Stop relying on Git for Windows being installed

This Action previously relied on Git for Windows to be installed in
`C:\Program Files\Git`. That is true for all hosted Windows agents.

However, this is not necessarily true for all self-hosted runners.

But there usually _is_ a Git for Windows we can rely on: The build
agent's own copy. Let's use that instead.

This fixes #472

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

src/git.ts

@@ -4,8 +4,15 @@ import {Octokit} from '@octokit/rest'
 import {delimiter} from 'path'
 import * as fs from 'fs'
 
-export const gitForWindowsUsrBinPath = 'C:/Program Files/Git/usr/bin'
-const gitExePath = 'C:/Program Files/Git/cmd/git.exe'
+// If present, do prefer the build agent's copy of Git
+const externalsGitDir = `${process.env.AGENT_HOMEDIRECTORY}/externals/git`
+const gitForWindowsRoot = 'C:/Program Files/Git'
+const gitRoot = fs.existsSync(externalsGitDir)
+  ? externalsGitDir
+  : gitForWindowsRoot
+
+export const gitForWindowsUsrBinPath = `${gitRoot}/usr/bin`
+const gitExePath = `${gitRoot}/cmd/git.exe`
 
 /*
  * It looks a bit ridiculous to use 56 workers on a build agent that has only