Update HEAD when checking out from git cache

HEAD may be different than what git cache has. This change ensures the
correct revision is used.

R=gavinmak@google.com

Bug: 1251783
Change-Id: I4909a4d52ef70ee806c285f24693eca5173003d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3174363
Auto-Submit: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -727,6 +727,11 @@ def _git_checkout(sln, sln_dir, revisions, refs, no_fetch_tags, git_cache_dir,
         git('remote', 'set-url', 'origin', mirror_dir, cwd=sln_dir)
         git('fetch', 'origin', cwd=sln_dir)
       git('remote', 'set-url', '--push', 'origin', url, cwd=sln_dir)
+
+      if revision == 'HEAD':
+        # Update HEAD since cache may point to a different rev
+        git('remote', 'set-head', 'origin', '-a', cwd=sln_dir)
+
       if pin:
         git('fetch', 'origin', pin, cwd=sln_dir)
       for ref in refs: