Do not try to fetch default revision for mirrors always
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 6856f98467aa5c98085cdee02587dbab984cebb1
  "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 <3459da914636082bf719c966e5ccf07342eba8c5@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>
Tested-by: Chirayu Desai <5551c6b7e33c2172ec3f3b3f40b4baf275be9066@gmail.com>
(cherry picked from commit f7b64e3350a622ee87e1927cdbc8d854a5696d85)

couchbasedeps/git-repo,lewixliu/git-repo,GerritCodeReview/git-repo,GerritCodeReview/git-repo,lewixliu/git-repo,couchbasedeps/git-repo