commit: cope with scissors lines in commit message

commit: cope with scissors lines in commit message

The diff and submodule shortlog appended to the commit message template
by 'git commit --verbose' are not stripped when the commit message
contains an indented scissors line.

When cleaning up a commit message with 'git commit --verbose' or
'--cleanup=scissors' the code is careful and triggers only on a pure
scissors line, i.e. a line containing nothing but a comment character, a
space, and the scissors cut.  This is good, because people can embed
scissors lines in the commit message while using 'git commit --verbose',
and the text they write after their indented scissors line doesn't get
deleted.

While doing so, however, the cleanup function only looks at the first
line matching the scissors pattern and if it doesn't start at the
beginning of the line, then the function just returns without performing
any cleanup.  This is wrong, because a "real" scissors line added by
'git commit --verbose' might follow, and in that case the diff and
submodule shortlog get included in the commit message.

Fix this by changing the scissors pattern to match only at the beginning
of the line, yet be careful to catch scissors on the first line as well.

Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>