If there is a branch named after a file in the repository root, the
command `git rev-list --count $branch ^$base` is ambiguous. This is
because `git` does not know whether $branch needs to be parsed as a
revision or a filename.

Adding a trailing `--` parameter to the command-line resolves this
ambiguity since when present, everything before the `--` cannot be
a filename and everything after `--` has to one.

From git documentation:

  Paths may need to be prefixed with -- to separate them from
  options or the revision range, when confusion arises.

Bug: none
Change-Id: Ieb10aa8701e12fc3c88d5f75ff624f61ee8d8aaa
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2475773Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>