pickaxe -G: terminate early on matching lines

Solve a long-standing item for "git log -Grx" of us e.g. finding "+
str" in the diff context and noting that we had a "hit", but xdiff
diligently continuing to generate and spew the rest of the diff at
us. This makes use of a new "early return" xdiff interface added by
preceding commits.

The TODO item (or, the NEEDSWORK comment) has been there since "git
log -G" was implemented. See f506b8e8b5 (git log/diff: add -G<regexp>
that greps in the patch text, 2010-08-23).

But now with the support added in the preceding changes to the
xdiff-interface we can return early. Let's assert the behavior of that
new early-return xdiff-interface by having a BUG() call here to die if
it ever starts handing us needless work again.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

xdiff-interface.h

@@ -27,6 +27,10 @@
  * doing so will currently make your early return indistinguishable
  * from an error internal to xdiff, xdiff itself will see that
  * non-zero return and translate it to -1.
+ *
+ * See "diff_grep" in diffcore-pickaxe.c for a trick to work around
+ * this, i.e. using the "consume_callback_data" to note the desired
+ * early return.
  */
 typedef int (*xdiff_emit_line_fn)(void *, char *, unsigned long);
 typedef void (*xdiff_emit_hunk_fn)(void *data,