Two small changes to the Git metadata read path. Neither has a user-visible
effect on any platform except for a malformed `.git` gitfile, described below.
1. resolveGitMetadataPath's third parameter becomes an options object
`{ platform?, wslDistro? }`. A caller that knows which distro wrote a pointer
can now say so, where previously only a WSL UNC base path could. The distro
encoded in the base path still outranks the caller's, and translation only
happens when the reading host is win32, so a caller-named distro cannot make
a POSIX host fabricate a Windows path. The UNC-base branch is exempt from
that gate because that spelling only exists on Windows. Main's other
contracts are verbatim: never null for a non-empty pointer, and a drvfs
pointer keeps its drive spelling even when a distro is named. Both production
call sites (repo-git-marker-scan.ts) pass no options, so they are unchanged.
2. The `.git` gitfile marker parse moves into one shared function,
parseGitdirMarkerPayload: `gitdir:` at the start of the file, payload
trimmed, empty payload rejected — git's own read_gitfile_gently rule.
resolve-git-dir.ts and repo-git-marker-scan.ts both call it; the latter had a
near-identical private copy and is behaviorally identical after the swap
(verified across twelve marker spellings; the only divergence, a
whitespace-only payload, already resolved to null one call further down).
Main's `/^gitdir:\s*(.+)\s*$/m` in resolve-git-dir captured trailing padding
into the path and honored a `gitdir:` line anywhere in the file.
Per-platform delta: none on macOS, Linux, native Windows, WSL, SSH, relay, or
folder workspaces. The wslDistro option is inert; this change adds no caller.
For a malformed `.git` gitfile, padding is now stripped (strict improvement), a
whitespace-only payload falls back to `<worktree>/.git`, and a `gitdir:` line
that is not the first line is no longer honored — a narrowing, since main could
return a working gitdir there. All four resolveGitDir consumers already degrade
through a catch, so that case reports no sparse state / conflict operation /
diff stamp rather than failing.
Six other hand-rolled `gitdir:` parsers remain, including the relay's SSH copy;
converging them is its own change.
