fix: fetch the project main an upstream lacks

A remote named upstream added as CONTRIBUTING said and never fetched made validate_pr --changed compare with the fork's main and exit 0 on nothing. It now fetches the project's main when the remote does not hold it. profile_sync names the fetch when --base is absent, and CONTRIBUTING adds git fetch upstream.

scripts/validate_pr.py

@@ -558,30 +558,35 @@ def _repo_key(url: str) -> str:
 
 
 def _project_main_line() -> str | None:
-    """The project's main line, fetched when no remote of this clone names it.
+    """The project's main line, fetched when this clone does not hold it.
 
     A fork's clone has origin alone. Once the commit is pushed to the fork's
     main, origin/main is HEAD and the diff against it was empty, so --changed
-    found nothing and exited 0. None leaves the upstream/ and origin/ lines
-    in charge: a remote of that name names the project, or the clone has no
-    remote at all and only its uncommitted files can be read.
+    found nothing and exited 0. A remote added as CONTRIBUTING says and never
+    fetched did the same: upstream/main did not exist and origin/main won.
+    None leaves the upstream/ and origin/ lines in charge when the clone has
+    no remote at all and only its uncommitted files can be read.
     """
     remotes = _git_lines("remote") or []
     if not remotes:
         return None
     project = {_repo_key(url) for url in PROJECT_URLS}
+    source = PROJECT_URLS[0]
     for remote in remotes:
         urls = _git_lines("remote", "get-url", "--all", remote) or []
         if any(_repo_key(url) in project for url in urls):
-            return None if remote in ("upstream", "origin") else f"{remote}/main"
+            if _git_lines("rev-parse", "--verify", "--quiet", f"{remote}/main"):
+                return f"{remote}/main"
+            source = remote
+            break
     fetched = subprocess.run(
-        ["git", "fetch", "--quiet", "--no-tags", PROJECT_URLS[0], "main"],
+        ["git", "fetch", "--quiet", "--no-tags", source, "main"],
         capture_output=True, text=True, check=False,
     )
     if fetched.returncode != 0:
         raise RuntimeError(
-            f"no remote of this clone is {PROJECT_URLS[0]}, and fetching its "
-            f"main failed: {fetched.stderr.strip()}"
+            f"this clone does not hold the main of {PROJECT_URLS[0]}, and "
+            f"fetching it from {source} failed: {fetched.stderr.strip()}"
         )
     return "FETCH_HEAD"