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/generate_readme.py

@@ -986,8 +986,9 @@ vermin --no-tips -t=3.8- --violations install.py         # install.py stays on P
 ```
 
 `upstream` is this repository as a remote of your fork
-(`git remote add upstream https://github.com/Abdess/retrobios`). A profile you
-only edit is not held to `--strict`: its older refs are reviewed by hand.
+(`git remote add upstream https://github.com/Abdess/retrobios`, then
+`git fetch upstream`). A profile you only edit is not held to `--strict`: its
+older refs are reviewed by hand.
 
 The full pipeline is not needed for a contribution: it rebuilds every pack
 in `dist/`, which is why maintainers run it before a release.

scripts/profile_sync.py

@@ -4355,6 +4355,10 @@ def branch_profiles(emulators_dir: str, base: str, added_only: bool) -> list[str
     branch point are not counted, and ends at the working tree, so a
     profile not yet committed is checked too. A rename reads as an addition.
     """
+    if subprocess.run(["git", "rev-parse", "--verify", "--quiet", base],
+                      capture_output=True, check=False).returncode:
+        remote = base.split("/", 1)[0] if "/" in base else "upstream"
+        raise SystemExit(f"ERROR: {base} is not in this clone: run git fetch {remote} first")
     merge_base = _git_lines(["merge-base", base, "HEAD"])[0]
     paths = _git_lines([
         "diff", "--name-only", "--no-renames",

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"