project: Avoid skipping fetches for shallow clones without .git/shallow

When optimizing fetches for projects with immutable revisions, the fetch
should not be skipped if the project is configured for a shallow clone
(depth > 0) but the .git/shallow file is missing. The absence of the
.git/shallow file means the repository is not a shallow clone, or the
shallow clone is incomplete, so a fetch is necessary to ensure the
revision is present.

Bug: 503081454
Change-Id: Ic3549612bcd69050a926652ee4e522c79ad8124c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/573821
Tested-by: Becky Siegel <beckysiegel@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Becky Siegel <beckysiegel@google.com>

project.py

@@ -1495,6 +1495,10 @@ class Project:
             and self._CheckForImmutableRevision(
                 use_superproject=use_superproject
             )
+            and (
+                not depth
+                or os.path.exists(os.path.join(self.gitdir, "shallow"))
+            )
         ):
             remote_fetched = True
             try:
@@ -2616,6 +2620,9 @@ class Project:
             if is_sha1 or tag_name is not None:
                 if self._CheckForImmutableRevision(
                     use_superproject=use_superproject
+                ) and (
+                    not depth
+                    or os.path.exists(os.path.join(self.gitdir, "shallow"))
                 ):
                     if verbose:
                         print(