Merge branch 'sh/submodule-summary-abbrev-fix'

The "git submodule summary" subcommand showed shortened commit object names by mechanically truncating them at 7-hexdigit, which has been improved to let "rev-parse --short" scale the length of the abbreviation with the size of the repository. * sh/submodule-summary-abbrev-fix: git-submodule.sh: shorten submodule SHA-1s using rev-parse