project: derive HEAD fallback from git's own default branch

When the default branch cannot be determined dynamically, repo fell back
to a hardcoded "refs/heads/master". This can point at a branch that does
not exist on the server, since projects increasingly default to "main".
Rather than swap one hardcoded name for another, ask git itself what it
would use.

Bug: 483758905
Change-Id: Ic66712acea98f8e548a2d6d8211865ee5c416b4d
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/612845
Tested-by: Brian Gan <brgan@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Commit-Queue: Brian Gan <brgan@google.com>

project.py

@@ -15,6 +15,7 @@
 import datetime
 import errno
 import filecmp
+import functools
 import glob
 import os
 import platform
@@ -4663,6 +4664,26 @@ class SyncBuffer:
         self._pending_failures = []
 
 
+@functools.lru_cache(maxsize=None)
+def _DefaultBranchFallback() -> str:
+    """Return the ref to use when remote default branch can't be resolved."""
+
+    def _git(args: List[str]) -> str:
+        p = GitCommand(
+            None,
+            args,
+            capture_stdout=True,
+            capture_stderr=True,
+            log_as_error=False,
+        )
+        return p.stdout.strip() if p.Wait() == 0 else ""
+
+    branch = _git(["var", "GIT_DEFAULT_BRANCH"]) or _git(
+        ["config", "--get", "init.defaultBranch"]
+    )
+    return f"refs/heads/{branch or 'master'}"
+
+
 class MetaProject(Project):
     """A special project housed under .repo."""
 
@@ -4676,7 +4697,7 @@ class MetaProject(Project):
             worktree=worktree,
             remote=RemoteSpec("origin"),
             relpath=".repo/%s" % name,
-            revisionExpr="refs/heads/master",
+            revisionExpr=_DefaultBranchFallback(),
             revisionId=None,
             groups=None,
         )
@@ -5134,9 +5155,9 @@ class ManifestProject(MetaProject):
                 if is_new:
                     default_branch = self.ResolveRemoteHead()
                     if default_branch is None:
-                        # If the remote doesn't have HEAD configured, default to
-                        # master.
-                        default_branch = "refs/heads/master"
+                        # If the remote doesn't have HEAD configured, fall back
+                        # to whatever git uses as its default branch.
+                        default_branch = _DefaultBranchFallback()
                     self.revisionExpr = default_branch
                 else:
                     self.PreSync()