A git timeout or failure is reported as "no-merge-base" or 409 "unknown-base" (and counted in unknown_base) instead of as a timeout or server error #69

What's wrong

MirrorQueries collapses every git failure into a negative answer:

• ContainsCommitAsync (GitBranchStateCache/Mirrors/MirrorQueries.cs:47-52) returns result.Succeeded. A timed-out or crashed cat-file -e therefore reads as "commit not in mirror".
• FindMergeBaseAsync (MirrorQueries.cs:72-78) returns null on any non-success. That includes TimedOut and exit 128 from a corrupt or locked repository.

BranchStateHandler then reports these to the client as data facts:

• :182-194: a false ContainsCommitAsync becomes 409 unknown-base and increments the unknown_base metric.
• :239-244: a null merge base becomes a branch entry with error: "no-merge-base".

The diff path already distinguishes these cases through DiffOutcome.Timeout. The commit check and merge-base queries don't.

Failure scenario

Reproduced at HEAD cbd20fc by adding a temporary timeout flag to ScriptedGit and two integration tests:

• merge-base times out (a large repo going over DiffTimeout):
  200 {"branches":[{"name":"origin/main",...,"mergeBase":null,"error":"no-merge-base"}],"partial":true}
• cat-file -e times out (over ProbeTimeout, or a corrupt mirror):
  409 {"error":"unknown-base","message":"The mirror does not hold that commit..."}

The client is told its histories are unrelated, or that its base commit was never pushed. A client following the documented contract might act on that, for example by pushing again or dropping the branch. On the server side, unknown_base rises, and README:169 names that counter as the one to alert on, so a service fault (slow disk, corrupt mirror) is misread as client behaviour.

Suggested fix

• Return a three-state result from both queries: Found / Absent / Failed(TimedOut). Treat only exit code 1 as absent: git merge-base exits 1 for unrelated histories, and git cat-file -e exits 1 for a missing object. Every other outcome is a failure.
• In BranchStateHandler:
  • Map a merge-base timeout to the branch-level timeout error the diff path already uses, and any other failure to a distinct error.
  • Map a failed commit check to 504 (timeout) or 502 (other), not 409, and don't increment unknown_base.

Acceptance criteria

• With merge-base timing out, the branch reports error: "timeout", not no-merge-base.
• With cat-file timing out or failing, the response is 5xx and unknown_base is unchanged.
• A genuinely missing commit (exit 1) still returns 409 unknown-base, and genuinely unrelated histories still report no-merge-base.