Document git log --abbrev-commit, as a kind of pretty option

Document git log --abbrev-commit, as a kind of pretty option

Documentation taken from paraphrased description of "--abbrev[=<n>]"
diff option, and from description of commit 5c51c985 introducing
this option.

Note that to change number of digits one must use "--abbrev=<n>",
which affects [also] diff output.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>