mailinfo: fix MIME multi-part message boundary handling

mailinfo: fix MIME multi-part message boundary handling

After finding a MIME multi-part message boundary line, the handle_body()
function is supposed to first flush any accumulated contents from the
previous part to the output stream.  However, the code mistakenly output
the boundary line it found.

The old code that used one global, fixed-length buffer line[] used an
alternate static buffer newline[] for keeping track of this accumulated
contents and flushed newline[] upon seeing the boundary; when 3b6121f
(git-mailinfo: use strbuf's instead of fixed buffers, 2008-07-13)
converted a fixed-length buffer in this program to use strbuf,these two
buffers were converted to "line" and "prev" (the latter of which now has a
much more sensible name) strbufs, but the code mistakenly flushed "line"
(which contains the boundary we have just found), instead of "prev".

This resulted in the first boundary to be output in front of the first
line of the message.

The rewritten implementation of handle_boundary() lost the terminating
newline; this would then result in the second line of the message to be
stuck with the first line.

The is_multipart_boundary() was designed to catch both the internal
boundary and the terminating one (the one with trailing "--"); this also
was broken with the rewrite, and the code in the handle_boundary() to
handle the terminating boundary was never triggered.

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