Don't fetch tags in presubmit

The presubmit step does not require tags so avoid them, as fetching
tags implicitly adds a lot of time to the fetch step for repos like
chromium with many many tags.

R=iannucci@chromium.org

Recipe-Nontrivial-Roll: build
Bug: 1019824
Change-Id: I741970b8c61bd88bafde6ef30b733e9bfc2c0a34
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1951510Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>

recipes/recipe_modules/presubmit/api.py

@@ -52,7 +52,8 @@ class PresubmitApi(recipe_api.RecipeApi):
     """
     # Expect callers to have already set up their gclient configuration.
 
-    bot_update_step = self.m.bot_update.ensure_checkout(timeout=3600)
+    bot_update_step = self.m.bot_update.ensure_checkout(
+        timeout=3600, no_fetch_tags=True)
     relative_root = self.m.gclient.get_gerrit_patch_root().rstrip('/')
 
     abs_root = self.m.context.cwd.join(relative_root)