[PATCH 4/5] diff: return line_prefix directly when possible

We may point our output_prefix callback to diff_output_prefix_callback()
in any of these cases:

  1. we have a user-provided line_prefix

  2. we have a graph prefix to show

  3. both (1) and (2)

The function combines the available elements into a strbuf and returns
its pointer.

In the case that we just have the line_prefix, though, there is no need
for the strbuf. We can return the string directly.

This is a minor optimization by itself, but also will allow us to clean
up some memory ownership issues on top.

Signed-off-by: Jeff King <redacted>
---
The old code did handle a case "0" where neither is set. But in that
case we would never set up the callback in the first place. So I didn't
think it was worth being defensive there.

 graph.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/graph.c b/graph.c
index c046f6285d..0d200ca77f 100644
--- a/graph.c
+++ b/graph.c

@@ -318,6 +318,9 @@ static const char *diff_output_prefix_callback(struct diff_options *opt, void *d
 
 	assert(opt);
 
+	if (!graph)
+		return opt->line_prefix;
+
 	strbuf_reset(&msgbuf);
 	if (opt->line_prefix)
 		strbuf_addstr(&msgbuf, opt->line_prefix);

-- 
2.47.0.rc1.384.g9f398d04fd