feat(gitcommit): add support for git commit --amend

- implement amend detection with is_amending() function
- add get_contextual_diff() to handle staged and amend scenarios
- update error messages to differentiate between staged and amend contexts
- modify commit command to use --amend flag when appropriate
- enhance notification messages to reflect commit vs amend actions

Author: jinzhongjia

lua/codecompanion/_extensions/gitcommit/buffer.lua

@@ -62,10 +62,13 @@ function Buffer._generate_and_insert_commit_message(bufnr)
     return
   end
 
-  -- Get staged changes
-  local diff = Git.get_staged_diff()
+  -- Get relevant changes (staged or amend)
+  local diff, context = Git.get_contextual_diff()
   if not diff then
-    vim.notify("No staged changes found. Please stage your changes first.", vim.log.levels.ERROR)
+    local msg = context == "no_changes"
+        and (Git.is_amending() and "No changes to amend" or "No staged changes found. Please stage your changes first.")
+      or "Failed to get git changes"
+    vim.notify(msg, vim.log.levels.ERROR)
     return
   end
 

lua/codecompanion/_extensions/gitcommit/git.lua

@@ -33,23 +33,91 @@ function Git.is_repository()
   return vim.v.shell_error == 0 and vim.trim(result) == "true"
 end
 
----Get git diff for staged changes
----@return string|nil diff The staged changes diff, nil if no changes or error
-function Git.get_staged_diff()
+---Check if currently in git commit --amend state
+---@return boolean
+function Git.is_amending()
   if not Git.is_repository() then
-    return nil
+    return false
   end
 
-  local diff = vim.fn.system("git diff --no-ext-diff --staged")
+  -- Check if COMMIT_EDITMSG exists and we're in a rebase/merge state
+  local git_dir = vim.fn.system("git rev-parse --git-dir"):gsub("\n", "")
   if vim.v.shell_error ~= 0 then
-    return nil
+    return false
+  end
+
+  -- Check for COMMIT_EDITMSG file which indicates we're editing a commit
+  local commit_editmsg = git_dir .. "/COMMIT_EDITMSG"
+  local stat = vim.uv.fs_stat(commit_editmsg)
+  if not stat then
+    return false
   end
 
-  if vim.trim(diff) == "" then
+  -- Additional check: see if we have HEAD commit (not initial commit)
+  local head_check = vim.fn.system("git rev-parse --verify HEAD")
+  return vim.v.shell_error == 0
+end
+
+---Get git diff for staged changes or last commit (for amend)
+---@return string|nil diff The changes diff, nil if no changes or error
+function Git.get_staged_diff()
+  if not Git.is_repository() then
     return nil
   end
 
-  return diff
+  -- First try to get staged changes
+  local staged_diff = vim.fn.system("git diff --no-ext-diff --staged")
+  if vim.v.shell_error == 0 and vim.trim(staged_diff) ~= "" then
+    return staged_diff
+  end
+
+  -- If no staged changes and we're in amend mode, get the last commit's changes
+  if Git.is_amending() then
+    local last_commit_diff = vim.fn.system("git diff --no-ext-diff HEAD~1")
+    if vim.v.shell_error == 0 and vim.trim(last_commit_diff) ~= "" then
+      return last_commit_diff
+    end
+
+    -- Fallback: if HEAD~1 doesn't exist (initial commit), show all files
+    local show_diff = vim.fn.system("git show --no-ext-diff --format= HEAD")
+    if vim.v.shell_error == 0 and vim.trim(show_diff) ~= "" then
+      return show_diff
+    end
+  end
+
+  return nil
+end
+
+---Get contextual diff based on current git state
+---@return string|nil diff The relevant diff, nil if no changes
+---@return string context The context of what diff represents
+function Git.get_contextual_diff()
+  if not Git.is_repository() then
+    return nil, "not_in_repo"
+  end
+
+  -- Check for staged changes first
+  local staged_diff = vim.fn.system("git diff --no-ext-diff --staged")
+  if vim.v.shell_error == 0 and vim.trim(staged_diff) ~= "" then
+    return staged_diff, "staged"
+  end
+
+  -- Check if we're amending
+  if Git.is_amending() then
+    -- Try to get the last commit's diff
+    local last_commit_diff = vim.fn.system("git diff --no-ext-diff HEAD~1")
+    if vim.v.shell_error == 0 and vim.trim(last_commit_diff) ~= "" then
+      return last_commit_diff, "amend_with_parent"
+    end
+
+    -- Fallback for initial commit amend
+    local show_diff = vim.fn.system("git show --no-ext-diff --format= HEAD")
+    if vim.v.shell_error == 0 and vim.trim(show_diff) ~= "" then
+      return show_diff, "amend_initial"
+    end
+  end
+
+  return nil, "no_changes"
 end
 
 ---Commit changes with the provided message
@@ -61,10 +129,16 @@ function Git.commit_changes(message)
     return false
   end
 
-  -- Check if there are staged changes
-  local diff = Git.get_staged_diff()
+  -- Check if there are changes to commit
+  local diff, context = Git.get_contextual_diff()
   if not diff then
-    vim.notify("No staged changes found. Please stage your changes first.", vim.log.levels.ERROR)
+    if context == "no_changes" then
+      if Git.is_amending() then
+        vim.notify("No changes to amend. The commit already exists.", vim.log.levels.WARN)
+      else
+        vim.notify("No staged changes found. Please stage your changes first.", vim.log.levels.ERROR)
+      end
+    end
     return false
   end
 
@@ -79,16 +153,23 @@ function Git.commit_changes(message)
   file:write(message)
   file:close()
 
-  -- Execute git commit
-  local cmd = string.format("git commit -F %s", vim.fn.shellescape(temp_file))
+  -- Execute appropriate git commit command
+  local cmd
+  if Git.is_amending() then
+    cmd = string.format("git commit --amend -F %s", vim.fn.shellescape(temp_file))
+  else
+    cmd = string.format("git commit -F %s", vim.fn.shellescape(temp_file))
+  end
+
   local result = vim.fn.system(cmd)
   local exit_code = vim.v.shell_error
 
   -- Clean up temporary file
   os.remove(temp_file)
 
   if exit_code == 0 then
-    vim.notify("Successfully committed changes!", vim.log.levels.INFO)
+    local action = Git.is_amending() and "amended" or "committed"
+    vim.notify(string.format("Successfully %s changes!", action), vim.log.levels.INFO)
     return true
   else
     local error_msg = vim.trim(result)

lua/codecompanion/_extensions/gitcommit/init.lua

@@ -16,10 +16,13 @@ function M.generate_commit_message()
     return
   end
 
-  -- Get staged changes
-  local diff = Git.get_staged_diff()
+  -- Get relevant changes (staged or amend)
+  local diff, context = Git.get_contextual_diff()
   if not diff then
-    vim.notify("No staged changes found. Please stage your changes first.", vim.log.levels.ERROR)
+    local msg = context == "no_changes"
+        and (Git.is_amending() and "No changes to amend" or "No staged changes found. Please stage your changes first.")
+      or "Failed to get git changes"
+    vim.notify(msg, vim.log.levels.ERROR)
     return
   end