[ Tool ] Fix "Error: Unable to find git in your PATH" when Command Processor AutoRun registry key is defined (#159424)

cmd.exe will read from the AutoRun registry key at launch and execute
the commands listed in the key. Since `FOR /F IN (command) ...` causes a
new terminal instance to start, the AutoRun commands will be run again,
possibly changing the current working directory. In this case, the `git
rev-parse HEAD` check run in `shared.bat` could fail, which we were
assuming meant `git` was not on the user's PATH.

This change ensures that `git rev-parse HEAD` will always run in
%FLUTTER_ROOT% and explicitly adds a separate check for `git` using
`WHERE git` to more accurately determine if `git` is on the PATH.

Fixes flutter/flutter#159018

---------

Co-authored-by: Andrew Kolos <andrewrkolos@gmail.com>

Author: bkonyi

bin/internal/shared.bat

@@ -52,18 +52,24 @@ GOTO :after_subroutine
     CALL "%bootstrap_path%"
   )
 
-  REM Check that git exists and get the revision
-  SET git_exists=false
+  REM Check that git exists and get the revision.
+  WHERE git >NUL 2>&1
+  IF "%ERRORLEVEL%" NEQ "0" (
+    REM Could not find git. Exit without /B to avoid retrying.
+    ECHO Error: Unable to find git in your PATH. && EXIT 1
+  )
   2>NUL (
-    PUSHD "%flutter_root%"
-    FOR /f %%r IN ('git rev-parse HEAD') DO (
-      SET git_exists=true
+    REM 'FOR /f' spawns a new terminal instance to run the command. If an
+    REM 'AutoRun' command is defined in the user's registry, that command could
+    REM change the working directory, and then we wouldn't be in the directory
+    REM we expect to be in. To prevent this, we need to 'PUSHD %FLUTTER_ROOT%'
+    REM before getting the git revision.
+    REM
+    REM See https://github.com/flutter/flutter/issues/159018
+    FOR /f %%r IN ('PUSHD %FLUTTER_ROOT% ^& $git rev-parse HEAD') DO (
       SET revision=%%r
     )
-    POPD
   )
-  REM If git didn't execute we don't have git. Exit without /B to avoid retrying.
-  if %git_exists% == false echo Error: Unable to find git in your PATH. && EXIT 1
   SET compilekey="%revision%:%FLUTTER_TOOL_ARGS%"
 
   REM Invalidate cache if: