[PATCH v2 0/3] commit: fix abbrev-sha regression

After c197702 (pretty: Respect --abbrev option), the summary output for
git-commit began to throw up non-abbreviated SHA-1s.

For example,

  $ git commit --allow-empty -m "empty"
  [master f869bf09c8eefee54094bb21387241aaf5f10324] empty

I also notice that this happens for merges after conflict resolutions.

This afflicts 'master', but not any release.

Changes from v1:

 - move cleanup commands into separate function in patch #1.
 - add a cleanup command to patch #1 to accomodate tests #20 and #22.
 - changed shell syntax in patch #2 (based on Junio's suggestion).
 - used a more "aggressive" fix in patch #3.

Contents:
[PATCH v2 1/3] t7502-commit: add tests for summary output
[PATCH v2 2/3] t7502-commit: add summary output tests for empty and merge commits
[PATCH v2 3/3] commit::print_summary(): set rev_info.always_show_header to 1