fix(stack): surface fetch_old_pr_heads failures instead of swallowing (#1315)

The exception handler around `fetch_old_pr_heads` previously did a
silent `pass`, leaving every revision-history entry stamped
"unknown" with no clue why. When a real failure happens (fork remote
without `refs/pull/N/head`, network error, missing permissions, …)
the user has no signal that anything went wrong — they only see the
symptom in the PR comment days later.

Log the underlying error in orange so the cause is visible at push
time. The exception text is run through `rich.markup.escape` so any
`[`/`]` in the git error message is rendered literally rather than
parsed as Rich markup. Behaviour stays non-fatal: classification
still falls back to "unknown" and the push proceeds.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Depends-On: #1316

Author: jd

mergify_cli/stack/push.py

@@ -24,6 +24,8 @@
 import sys
 import typing
 
+import rich.markup
+
 from mergify_cli import console
 from mergify_cli import console_error
 from mergify_cli import utils
@@ -582,8 +584,16 @@ async def stack_push(
             with console.status("Fetching old PR heads for comparison..."):
                 try:
                     await fetch_old_pr_heads(remote, updated_pr_numbers)
-                except utils.CommandError:
-                    pass  # Non-fatal: change type will be "unknown"
+                except utils.CommandError as exc:
+                    # Non-fatal: change type will be "unknown" — but surface
+                    # the underlying error so the user can fix it. Escape the
+                    # exception text since it can contain `[`/`]` that Rich
+                    # would otherwise interpret as markup tags.
+                    console.log(
+                        f"[orange]Could not fetch old PR heads; revision-history "
+                        f"change types will fall back to 'unknown': "
+                        f"{rich.markup.escape(str(exc))}[/]",
+                    )
 
             # Detect change types before force-push overwrites refs
             change_types: dict[str, str] = {}