superproject: Added --depth=1 argument to git fetch command.

Tested the code with the following commands.

$ ./run_tests -v

Without --depth

$ time repo_dev init -u sso://googleplex-android.git.corp.google.com/platform/manifest -b master --repo-rev=main --use-superproject
real    6m48.086s
user    3m27.281s
sys     1m1.386s

With --depth=1
$ time repo_dev init -u sso://googleplex-android.git.corp.google.com/platform/manifest -b master --repo-rev=main --use-superproject

real    2m49.637s
user    2m51.458s
sys     0m39.108s

From dwillemsen@:
 "For me it's the difference between 9m28s using the current code and
  16s using --depth=1 while fetching the superproject."

Bug: [google internal] b/180451672
Bug: https://crbug.com/gerrit/13707
Change-Id: I1c3e4aef4414c4e9dd259fb6e4619da0421896b0
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/300922


Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Raman Tenneti <rtenneti@google.com>