Fix line directly below insertion not incremented

The Vim help for `listener_add()` states this:

> When lines are inserted the values are:
>     lnum	line above which the new line is added
>     ...

where line numbers are in the context of before the change. All lines
greater than or equal to lnum should therefore be incremented, and not
only those strictly greater.

Author: axelf4

plugin/strip_trailing_whitespace.vim

@@ -172,7 +172,7 @@ function s:Listener(bufnr, start, end, added, changes) abort
 	" Adjust line numbers
 	let b:root = s:Splay(b:root, a:start)
 	if b:root isnot s:null
-		if b:root.key > a:start
+		if b:root.key >= a:start
 			let b:root.key += a:added
 			if b:root.left isnot s:null | let b:root.left.key -= a:added | endif
 		elseif b:root.right isnot s:null

test/test.vim

@@ -1,20 +1,29 @@
-function Test_OneLineUnchanged() abort
+function s:TestEdits(original, EditCb, expected) abort
 	let fname = tempname()
-	let contents = ['line1 ']
-	call writefile(contents, fname)
+	call writefile(a:original, fname)
 	silent execute 'edit' fname
+	call a:EditCb()
 	silent write
-	call assert_equal(['line1 '], readfile(fname))
+	call assert_equal(a:expected, readfile(fname))
 	%bwipeout!
 endfunction
 
+function Test_OneLineUnchanged() abort
+	function! s:EditCb() abort
+	endfunction
+	call s:TestEdits(['foo '], function('s:EditCb'), ['foo '])
+endfunction
+
 function Test_OneLineChanged() abort
-	let fname = tempname()
-	let contents = ['line1 ']
-	call writefile(contents, fname)
-	silent execute 'edit' fname
-	normal! rf
-	silent write
-	call assert_equal(['fine1'], readfile(fname))
-	%bwipeout!
+	function! s:EditCb() abort
+		normal! rf
+	endfunction
+	call s:TestEdits(['line1 '], function('s:EditCb'), ['fine1'])
+endfunction
+
+function Test_AddLineAboveChange() abort
+	function! s:EditCb() abort
+		normal! rzO
+	endfunction
+	call s:TestEdits(['foo '], function('s:EditCb'), ['', 'zoo'])
 endfunction