format-patch: preserve subject newlines with -k

In older versions of git, we used rfc822 header folding to
indicate that the original subject line had multiple lines
in it.  But since a1f6baa (format-patch: wrap long header
lines, 2011-02-23), we now use header folding whenever there
is a long line.

This means that "git am" cannot trust header folding as a
sign from format-patch that newlines should be preserved.
Instead, format-patch needs to signal more explicitly that
the newlines are significant.  This patch does so by
rfc2047-encoding the newlines in the subject line. No
changes are needed on the "git am" end; it already decodes
the newlines properly.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

log-tree.c

diff --git a/log-tree.c b/log-tree.c
index 0d8cc7a..0c41789 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -504,6 +504,7 @@
 	ctx.date_mode = opt->date_mode;
 	ctx.abbrev = opt->diffopt.abbrev;
 	ctx.after_subject = extra_headers;
+	ctx.preserve_subject = opt->preserve_subject;
 	ctx.reflog_info = opt->reflog_info;
 	ctx.fmt = opt->commit_format;
 	pretty_print_commit(&ctx, commit, &msgbuf);