[RFC/PATCH] pretty.c: %S format specifier for source ref

Currently, there is no way to combine --source with a format string for
log and friends.

Introduce a %S format specifier which outputs the source ref if --source
is given also. This is analogous to specfiers like %gs which are empty
unless used with -g.

Signed-off-by: Michael J Gruber <redacted>
---
Notes:
    I'm not sure whether it's worth introducing pretty_ctx.show_source to
    keep track of the use of --source. commit->util is possibly used by
    other codepaths also. Even without this patch, --cherry-pick --source
    does not work (in the sense that there is an empty source output) because
    of this.
    
    RFC obviously also so that I don't have to write doc and tests just yet :)

 pretty.c |    4 ++++
 1 files changed, 4 insertions(+), 0 deletions(-)

diff --git a/pretty.c b/pretty.c
index 8b18efd..146320f 100644
--- a/pretty.c
+++ b/pretty.c

@@ -867,6 +867,10 @@ static size_t format_commit_one(struct strbuf *sb, const char *placeholder,
 	case 'd':
 		format_decoration(sb, commit);
 		return 1;
+	case 'S':
+		if (/* revs->show_source && */commit->util)
+			strbuf_addstr(sb, (char *) commit->util);
+		return 1;
 	case 'g':		/* reflog info */
 		switch(placeholder[1]) {
 		case 'd':	/* reflog selector */

-- 
1.7.1.511.gfbed4