Set core.trustctime=false during git checkout operations.

This should be faster and more general than running
`git index --refresh` over all git repositories we can find.

See "Proposal: Setting core.trustctime=false on Chromium's bots"
on infra-dev (linked to from bug 330461 comment 45).

Bug: 330461,642711,746763,740301.
Change-Id: I7c35cffa107d46d9cf98f0e3e813ef2ab98df5fc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1809419
Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Nico Weber <thakis@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -846,20 +846,20 @@ def ensure_checkout(solutions, revisions, first_sln, target_os, target_os_only,
   for solution_name in list(solution_dirs):
     gc_revisions[solution_name] = 'unmanaged'
 
-  with git_config_if_not_set('user.name', 'chrome-bot'):
-    with git_config_if_not_set('user.email', 'chrome-bot@chromium.org'):
-      # Let gclient do the DEPS syncing.
-      # The branch-head refspec is a special case because its possible Chrome
-      # src, which contains the branch-head refspecs, is DEPSed in.
-      gclient_output = gclient_sync(
-          BRANCH_HEADS_REFSPEC in refs,
-          TAGS_REFSPEC in refs,
-          gc_revisions,
-          break_repo_locks,
-          disable_syntax_validation,
-          patch_refs,
-          gerrit_reset,
-          gerrit_rebase_patch_ref)
+  with git_config_if_not_set('user.name', 'chrome-bot'), \
+       git_config_if_not_set('user.email', 'chrome-bot@chromium.org'):
+    # Let gclient do the DEPS syncing.
+    # The branch-head refspec is a special case because it's possible Chrome
+    # src, which contains the branch-head refspecs, is DEPSed in.
+    gclient_output = gclient_sync(
+        BRANCH_HEADS_REFSPEC in refs,
+        TAGS_REFSPEC in refs,
+        gc_revisions,
+        break_repo_locks,
+        disable_syntax_validation,
+        patch_refs,
+        gerrit_reset,
+        gerrit_rebase_patch_ref)
 
   # Now that gclient_sync has finished, we should revert any .DEPS.git so that
   # presubmit doesn't complain about it being modified.
@@ -1154,21 +1154,14 @@ def main():
 
   solutions_printer(git_slns)
 
-  # Creating hardlinks during a build can interact with git reset in
-  # unfortunate ways if git's index isn't refreshed beforehand. (See
-  # crbug.com/330461#c13 for an explanation.)
-  try:
-    call_gclient('recurse', '-v', 'git', 'update-index', '--refresh')
-  except SubprocessFailed:
-    # Failure here (and nowhere else) may have adverse effects on the
-    # compile time of the build but shouldn't affect its ability to
-    # successfully complete.
-    print 'WARNING: Failed to update git indices.'
-
   try:
     # Dun dun dun, the main part of bot_update.
-    revisions, step_text = prepare(options, git_slns, active)
-    checkout(options, git_slns, specs, revisions, step_text)
+    # gn creates hardlinks during the build. By default, this makes
+    # `git reset` overwrite the sources of the hardlinks, which causes
+    # unnecessary rebuilds. (See crbug.com/330461#c13 for an explanation.)
+    with git_config_if_not_set('core.trustctime', 'false'):
+      revisions, step_text = prepare(options, git_slns, active)
+      checkout(options, git_slns, specs, revisions, step_text)
 
   except PatchFailed as e:
     # Return a specific non-zero exit code for patch failure (because it is