project: centralize safe branch and commit resolution

Resolve commit-ish values through one typed helper using rev-parse
--verify --quiet and, on Git 2.30+, --end-of-options. Reject option-like
revisions on older clients, reuse the helper for project and manifest
resolution, and validate user branch names with check-ref-format
--branch.

Bug: 553599402
Change-Id: I2dda49ff3e781cec14d3b84263a8dace06b4073f
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/623182
Tested-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Gavin Mak <gavinmak@google.com>
Reviewed-by: Brian Gan <brgan@google.com>

subcmds/start.py

@@ -18,7 +18,7 @@ from typing import NamedTuple
 from command import Command
 from command import DEFAULT_LOCAL_JOBS
 from error import RepoExitError
-from git_command import git
+from git_command import IsValidBranchName
 from git_config import IsImmutable
 from progress import Progress
 from repo_logging import RepoLogger
@@ -75,7 +75,7 @@ revision specified in the manifest.
             self.Usage()
 
         nb = args[0]
-        if not git.check_ref_format("heads/%s" % nb):
+        if not IsValidBranchName(nb):
             self.OptionParser.error("'%s' is not a valid name" % nb)
 
     @classmethod