perf: batch branch commit-message lookups in shell completion

[tksm]

Summary

git wt's shell completion previously forked git log -1 once per branch to fetch each branch's commit subject, which became slow in repositories with many local or remote-tracking branches. This change batches the lookup into a single git for-each-ref call. In a 500-branch reproduction, completion drops from ~2.2s to ~0.12s (~20x faster).

Reproduction

The reproduction below uses local branches only for simplicity. git-wt is the released binary and git-wt-fixed is built from this branch.

tmp=$(mktemp -d) && cd "$tmp"
git init -q && git commit -q --allow-empty -m init
for i in $(seq 1 500); do git branch "b-$i"; done

time git-wt __complete "" >/dev/null

Completion ended with directive: ShellCompDirectiveNoFileComp
git-wt __complete "" > /dev/null  1.01s user 0.88s system 84% cpu 2.227 total

With the fix applied:

time git-wt-fixed __complete "" >/dev/null

Completion ended with directive: ShellCompDirectiveNoFileComp
git-wt-fixed __complete "" > /dev/null  0.05s user 0.06s system 87% cpu 0.119 total

I confirmed the completion output is identical before/after.

diff <(git-wt __complete "" 2>/dev/null) <(git-wt-fixed __complete "" 2>/dev/null)
# (no output → outputs are identical)