merge-ort: format messages slightly different for use in headers

When users run
    git show --remerge-diff $MERGE_COMMIT
or
    git log -p --remerge-diff ...
stdout is not an appropriate location to dump conflict messages, but we
do want to provide them to users.  We will include them in the diff
headers instead...but for that to work, we need for any multiline
messages to replace newlines with both a newline and a space.  Add a new
flag to signal when we want these messages modified in such a fashion,
and use it in path_msg() to modify these messages this way.  Also, allow
a special prefix to be specified for these headers.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.h

@@ -46,6 +46,8 @@ struct merge_options {
 	/* miscellaneous control options */
 	const char *subtree_shift;
 	unsigned renormalize : 1;
+	unsigned record_conflict_msgs_as_headers : 1;
+	const char *msg_header_prefix;
 
 	/* internal fields used by the implementation */
 	struct merge_options_internal *priv;