Introduce GIT_TRACE_PERFORMANCE

GIT_TRACE_PERFORMANCE shows invocation time of each subcommands.
https://git-scm.com/book/en/v2/Git-Internals-Environment-Variables

This is tested on
https://ci.chromium.org/swarming/task/3d605b8aab1a5a10?server=chromium-swarm.appspot.com
bot_update ran successfully.

Bug: 840592
Change-Id: I65ef733d1473f9f0f7e0109b0f727595766021b6
Reviewed-on: https://chromium-review.googlesource.com/1053028
Commit-Queue: Takuto Ikuta <tikuta@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -675,7 +675,10 @@ def _git_checkout(sln, sln_dir, revisions, shallow, refs, git_cache_dir,
   env = {}
   if url == CHROMIUM_SRC_URL or url + '.git' == CHROMIUM_SRC_URL:
     # This is for performance investigation of `git fetch` in chromium/src.
-    env = {'GIT_TRACE': 'true'}
+    env = {
+        'GIT_TRACE': 'true',
+        'GIT_TRACE_PERFORMANCE': 'true',
+    }
 
   # Step 1: populate/refresh cache, if necessary.
   pin = get_target_pin(name, url, revisions)