examples/commit: use --abbrev for commit summary

examples/commit: use --abbrev for commit summary

After v1.7.1.1~17^2~3 (pretty: Respect --abbrev option, 2010-05-03),
plumbing users do not abbreviate %h hashes by default any more.

Noticed while investigating the bug fixed by v1.7.1.1~17^2
(commit::print_summary(): don't use format_commit_message(),
2010-06-12).

Cc: Will Palmer <wmpalmer@gmail.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>