diff: move show_interdiff() from its own file to diff-lib

show_interdiff() is a relatively small function and not likely to grow
larger or more complicated. Rather than dedicating an entire source file
to it, relocate it to diff-lib.c which houses other "take two things and
compare them" functions meant to be re-used but not so low-level as to
reside in the core diff implementation.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -600,6 +600,8 @@ int index_differs_from(struct repository *r, const char *def,
 		       const struct diff_flags *flags,
 		       int ita_invisible_in_index);
 
+void show_interdiff(struct rev_info *, int indent);
+
 /*
  * Fill the contents of the filespec "df", respecting any textconv defined by
  * its userdiff driver.  The "driver" parameter must come from a