fix(commit): tolerate initial-commit repos in branch-name lookup (#844)

[gfargo]

Closes #844.

Summary

coco commit ran the full diff pipeline (collect → pre-process → consolidate → generate) and then crashed with fatal: ambiguous argument 'HEAD' when the post-summary step asked for the current branch via git rev-parse --abbrev-ref HEAD. On an initial-commit repo (fresh git init, no commits yet), HEAD doesn't resolve via rev-parse and the call fails fatally — but by that point the pipeline has already burned several minutes of LLM calls summarizing the staged changes, so the user pays the full cost and gets nothing back.

Fix

git symbolic-ref --short HEAD still reports the configured initial branch name even with no commits, so getCurrentBranchName now falls through to it on rev-parse failure and to an empty string only if both calls fail. Every caller already handles an empty branch context as "no branch info", so the no-HEAD case now runs the commit message generation to completion instead of aborting after the heavy lifting is done.

Test plan

• npm run lint
• npm run test:jest (1199 tests pass — the previous test that asserted the helper throws was replaced with five new tests covering the rev-parse path, the symbolic-ref fallback, output trimming, the both-fail empty-string path, and confirmation that symbolic-ref isn't called when rev-parse succeeds)
• npm run build
• npm run test:cli
• Manual: mkdir foo && cd foo && git init && touch a.txt && git add . && coco commit -a 'initial commit' --verbose — pipeline should complete and present a draft instead of crashing at the end

Follow-up

Performance work (4-minute pipeline on a 43-file initial commit) is tracked separately as #845. The crash fix here makes that performance cost no longer wasted, but the pipeline still needs the optimization pass.