rebase --abort: improve reflog message

rebase --abort: improve reflog message

When aborting a rebase the reflog message looks like

	rebase (abort): updating HEAD

which is not very informative. Improve the message by mentioning the
branch that we are returning to as we do at the end of a successful
rebase so it looks like.

	rebase (abort): returning to refs/heads/topic

If GIT_REFLOG_ACTION is set in the environment we no longer omit
"(abort)" from the reflog message. We don't omit "(start)" and
"(finish)" when starting and finishing a rebase in that case so we
shouldn't omit "(abort)".

Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>