perf(vcs): reduce git process count in git fallback from 5 to 3

[plttn]

Summary

Ports the git-subprocess-reduction optimization from IlanCosman/tide#663 (by @lgeiger, still open upstream) into this fork's git fallback path in functions/_tide_item_vcs.fish.

When git/jj got unified into _tide_item_vcs (v7.0.13), the git-handling logic was carried over verbatim from the old pre-#663 _tide_item_git.fish — 5 git processes per render (git branch --show-current, git rev-parse --git-dir --is-inside-git-dir, git status --porcelain, git stash list, git rev-list --count --left-right @{upstream}...HEAD). This never got the upstream fix since that PR hasn't merged.

Adding --branch to git status --porcelain puts a header line first in the output (e.g. ## main...origin/main [ahead 2, behind 1]) that already contains the branch name and ahead/behind counts, eliminating two of the five calls:

• Branch name is now parsed from that header (falling back to the old branch/tag/detached-HEAD cascade only when the header doesn't yield a usable name, e.g. detached HEAD)
• Ahead/behind come from a regex lookbehind on the same header instead of a separate git rev-list call
• git stash list stays, since git status can't report stash count

Result: 3 git processes instead of 5, same observable output. Only the git fallback path is touched — the jj path (_tide_internal_jj_git) is untouched.

Test plan

• mise run lint / fish_indent --check pass
• mise run test — full suite passes, including _tide_item_vcs.test.fish (branch names, detached HEAD, tags, ahead/behind, stash, staged/dirty/untracked, submodules, massive-status-repo)
• Updated the massive-status-repo test fixture to mock --branch and return a synthetic ## main header line, matching upstream's own test update in #663

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