Reuse the request-scoped base git repository in `ToAPIPullRequest`,
`ToPayloadPullRequest` and `ToAPIPullRequests` instead of reopening it
for each conversion, so conversions share the request's `cat-file`
process.
- Same-repo PRs reuse the base repository for the head instead of
opening it a second time.
- Drop a redundant second open of the base repository.
- Fork head repositories stay opened per call. A request-cached fork
keeps its `cat-file` running until the request ends, which blocks
deleting the fork on Windows and keeps one process per fork alive in
batch status changes.
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Co-authored-by: silverwind <me@silverwind.io>
fca64e7330