prose_lint.py Reads the Diff Prefix From Host Config, So Four Ordinary Settings Empty the Scope

[ptr727]

Problem

changed_lines in .github/actions/prose-gate/prose_lint.py keys diff scope off a +++ header it expects to begin b/. That prefix is not a property of git's output, it is a default the host's configuration can change, and the gate reads whichever the host has rather than pinning its own.

Four ordinary settings each empty the scope:

Setting	What the header becomes
diff.noprefix=true	+++ file.md, no prefix at all
diff.mnemonicPrefix=true	+++ w/file.md, the prefix varying by source
diff.dstPrefix=y/	+++ y/file.md
diff.external=<command>	no diff body at all

An empty scope is falsy, so the if scope and not matched refusal in main never fires. The run prints scope: 0 of N file(s) read and exits 0.

Evidence

Constructed: a repository holding one file with a duplicated word, one changed line.

settings unset          exit 1, the violation reported
diff.mnemonicPrefix     scope: 0 of 1 file(s) read, exit 0
diff.noprefix           scope: 0 of 1 file(s) read, exit 0
diff.dstPrefix=y/       scope: 0 of 1 file(s) read, exit 0
diff.external=<cmd>     scope: 0 of 1 file(s) read, exit 0

Confirmed present at a3cd9c7, so this is not a regression from #1574.

Reach

diff.mnemonicPrefix is an ordinary developer setting, so a maintainer carrying it gets a clean verdict from every local pre-push run of the gate. A CI runner carries none of them, so CI is unaffected and nothing here would notice.

Suggested fix

Pin what the parse depends on rather than inheriting it, the way #1574 pinned core.quotePath for the same reason in the same function: --no-ext-diff --src-prefix=a/ --dst-prefix=b/ on the same invocation. Verified to restore +++ b/ under all four settings.

TestDiffScopeReachesAQuotedName pins core.quotePath in its fixture and should pin these alongside it, since its own stated rationale covers them.

Found by a local review pass while fixing #1538 in #1574.