xdiff-interface: export comparing and hashing strings

This will turn out to be useful in a later patch.

xdl_recmatch is exported in xdiff/xutils.h, to be used by various
xdiff/*.c files, but not outside of xdiff/. This one makes it available
to the outside, too.

While at it, add documentation.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>