Fix shallow clones when upstream attribute is present

The _CheckForImmutableRevision method was modified in commit 0e776a58 to
include upstream branch validation for superproject scenarios. However,
this change inadvertently broke shallow clones when both clone-depth and
upstream attributes are specified in regular (non-superproject)
manifests.

Issue: When upstream is present, _CheckForImmutableRevision performs two
additional checks: 1. git rev-list on the upstream reference 2. git
merge-base --is-ancestor between revision and upstream

In shallow clones, the upstream branch history may not be available
locally, causing these checks to fail. This triggers the retry mechanism
that removes depth limitations, effectively converting shallow clones to
full clones, resulting in excessive disk usage.

Fix: Make upstream validation conditional on superproject usage. This
preserves the original superproject fix while restoring the method's
original behavior for regular scenarios - checking only if the immutable
revision (SHA1/tag) exists locally.

Note: The SetRevisionId method from the same commit 0e776a58 is left
unchanged as it only stores upstream information (no git operations),
which is beneficial for preserving branch context for commands like
'repo start' without causing fetch-related issues.

The fix ensures that manifests with both clone-depth and upstream work
correctly in non-superproject scenarios, maintaining shallow clone
efficiency and reducing disk usage.

Bug: b/427093249
Change-Id: I00acd4c61b179cd2abf796c2fecb7a2f38016a18
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/493883
Tested-by: Krzysztof Wesolowski <krzysztof.wesolowski@volvocars.com>
Commit-Queue: Krzysztof Wesolowski <krzysztof.wesolowski@volvocars.com>
Reviewed-by: Mike Frysinger <vapier@google.com>
Reviewed-by: Kamaljeet Maini <kamaljeet@google.com>
Reviewed-by: Xin Li <delphij@google.com>

project.py

@@ -2415,7 +2415,9 @@ class Project:
             # throws an error.
             revs = [f"{self.revisionExpr}^0"]
             upstream_rev = None
-            if self.upstream:
+
+            # Only check upstream when using superproject.
+            if self.upstream and self.use_superproject:
                 upstream_rev = self.GetRemote().ToLocal(self.upstream)
                 revs.append(upstream_rev)
 
@@ -2427,7 +2429,9 @@ class Project:
                 log_as_error=False,
             )
 
-            if self.upstream:
+            # Only verify upstream relationship for superproject scenarios
+            # without affecting plain usage.
+            if self.upstream and self.use_superproject:
                 self.bare_git.merge_base(
                     "--is-ancestor",
                     self.revisionExpr,