diff: return const char from output_prefix callback

diff: return const char from output_prefix callback

The diff_options structure has an output_prefix callback for returning a
prefix string, but it does so by returning a pointer to a strbuf.

This makes the interface awkward. There's no reason the callback should
need to use a strbuf, and it creates questions about whether the
ownership of the resulting buffer should be transferred to the caller
(it should not be, but a recent attempt to clean up this code led to a
double-free in some cases).

The one advantage we get is that the strbuf contains a ptr/len pair, so
we could in theory have a prefix with embedded NULs. But we can observe
that none of the existing callbacks would ever produce such a NUL (they
are usually just indentation or graph symbols, and even the
"--line-prefix" option takes a NUL-terminated string).

And anyway, only one caller (the one in log_tree_diff_flush) actually
looks at the strbuf length. In every other case we use a helper function
which discards the length and just returns the NUL-terminated string.

So let's just have the callback return a "const char *" pointer. It's up
to the callbacks themselves if they want to use a strbuf under the hood.
And now the caller in log_tree_diff_flush() can just use the helper
function along with everybody else. That lets us even simplify out the
function pointer check, since the helper returns an empty string
(technically this does mean we'll sometimes issue an empty fputs() call,
but I don't think this code path is hot enough to care about that).

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