Add log.abbrevCommit config variable

Add log.abbrevCommit config variable

Add log.abbrevCommit config variable as a convenience for users who
often use --abbrev-commit with git log and friends. Allow the option
to be overridden with --no-abbrev-commit. Per 635530a2fc and 4f62c2bc57,
the config variable is ignored when log is given "--pretty=raw".

(Also, a drive-by spelling correction in git log's short help.)

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>