Do not try to fetch default revision for mirrors always

* Mirrors may contain multiple projects, some of which may not
  always contain the default revision.
* Only fetch the default revision explicitly if
  '--current-branch' is set.
* Fixes breakage casued by
  commit 6856f984
  "Fix repo mirror with --current-branch"

Bug: https://crbug.com/gerrit/12274
Change-Id: Iaafabe2992f76f3644b841f24245d3e19c9515a9
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253093


Reviewed-by: Kuang-che Wu <kcwu@chromium.org>
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Chirayu Desai <chirayudesai1@gmail.com>
(cherry picked from commit f7b64e33)