git-commit: only append a newline to -m mesg if necessary

git-commit: only append a newline to -m mesg if necessary

Currently, git will append two newlines to every message supplied via
the -m switch.  The purpose of this is to allow -m to be supplied
multiple times and have each supplied string become a paragraph in the
resulting commit message.

Normally, this does not cause a problem since any trailing newlines will
be removed by the cleanup operation.  If cleanup=verbatim for example,
then the trailing newlines will not be removed and will survive into the
resulting commit message.

Instead, let's ensure that the string supplied to -m is newline terminated,
but only append a second newline when appending additional messages.

Fixes the test in t7502.

Signed-off-by: Brandon Casey <drafnel@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>