diffcore-pickaxe: unify code for log -S/-G

diffcore-pickaxe: unify code for log -S/-G

The logic flow of has_changes() used for "log -S" and diff_grep()
used for "log -G" are essentially the same.  See if we have both
sides that could be different in any interesting way, slurp the
contents in core, possibly after applying textconv, inspect the
contents, clean-up and report the result.  The only difference
between the two is how "inspect" step works.

Unify this codeflow in a helper, pickaxe_match(), which takes a
callback function that implements the specific "inspect" step.

After removing the common scaffolding code from the existing
has_changes() and diff_grep(), they each becomes such a callback
function suitable for passing to pickaxe_match().

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