rebase: try not to munge commit log message

rebase: try not to munge commit log message

This makes rebase/am keep the original commit log message
better, even when it does not conform to "single line paragraph
to say what it does, then explain and defend why it is a good
change in later paragraphs" convention.

This change is a two-edged sword.  While the earlier behaviour
would make such commit log messages more friendly to readers who
expect to get the birds-eye view with oneline summary formats,
users who primarily use git as a way to interact with foreign
SCM systems would not care much about the convenience of oneline
git log tools, but care more about preserving their own
convention.  This changes their commits less useful to readers
who read them with git tools while keeping them more consistent
with the foreign SCM systems they interact with.

Signed-off-by: Junio C Hamano <gitster@pobox.com>