am: don't call mailinfo if $rebasing

am: don't call mailinfo if $rebasing

Since 5e835ca (rebase: do not munge commit log message, 2008-04-16),
'git am --rebasing' no longer gets the commit log message from the
patch, but reads it from the commit identified by the "From " header
line. From 43c2325 (am: use get_author_ident_from_commit instead of
mailinfo when rebasing, 2010-06-16), it also gets the author name,
email and date from the commit. Now that the final part of the patch
-- the patch body itself -- is also read from the commit, there is no
longer a need to call 'git mailinfo' to extract any of these parts
while --rebasing.

Sugested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>