Use the gerrit host when setting gitiles commit.

Currently, the --revision flag to git cl try will cause a gitiles commit
to be set that uses the provided revision, the gerrit host and project
of the CL to test and the target ref of the CL to test. Since the flag
is intended to control the version of source to check out and have the
patch applied to, using the gerrit host instead of the gitiles host will
result in gitiles commits that won't match the repo_path_map in gclient
configs.

Change-Id: Ie391cc9c636f3a9c87116dbb781267031569e67b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3907186
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Garrett Beaty <gbeaty@google.com>