checkout, commit: remove confusing assignments to rev.abbrev

checkout, commit: remove confusing assignments to rev.abbrev

Since they do not precede setup_revisions, these assignments of 0 to
rev.abbrev have no effect.

v1.7.1.1~17^2~3 (2010-05-03) taught the log --format=%h machinery
to respect --abbrev instead of always abbreviating, so we have to pay
attention to the abbrev setting now.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>