diff: return line_prefix directly when possible

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