Avoid picking up a git.exe from the current directory

When running Git's test suite, there is a `git.exe` in the current
directory, and trying to use it fails because it won't look in the
correct place for `git-remote-https.exe`. This is the symptom:

	⏷ Cloning git-sdk-64
	  Notice: Cloning https://github.com/git-for-windows/git-sdk-64 to .tmp
	  git: 'remote-https' is not a git command. See 'git --help'.

Let's fix this bug by always specifying the full path to the Git
executable we intend to be used.

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

Author: dscho

src/git.ts

@@ -4,6 +4,7 @@ import {Octokit} from '@octokit/rest'
 import {delimiter} from 'path'
 
 const gitForWindowsUsrBinPath = 'C:/Program Files/Git/usr/bin'
+const gitExePath = 'C:/Program Files/Git/cmd/git.exe'
 
 /*
  * It looks a bit ridiculous to use 56 workers on a build agent that has only
@@ -24,7 +25,7 @@ async function clone(
 ): Promise<void> {
   if (verbose) core.notice(`Cloning ${url} to ${destination}`)
   const child = spawn(
-    'git.exe',
+    gitExePath,
     [
       'clone',
       '--depth=1',
@@ -109,7 +110,7 @@ export async function getViaGit(
       if (flavor === 'full') {
         core.startGroup(`Checking out ${repo}`)
         child = spawn(
-          'git.exe',
+          gitExePath,
           [`--git-dir=.tmp`, 'worktree', 'add', outputDirectory, head_sha],
           {
             env: {