diff: futureproof "stop feeding the backend early" logic

Refactor the "do not stop feeding the backend early" logic into a small
helper function and use it in both run_diff_files() and diff_tree() that
has the stop-early optimization. We may later add other types of diffcore
transformation that require to look at the whole result like diff-filter
does, and having the logic in a single place is essential for longer term
maintainability.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff.h

@@ -170,6 +170,8 @@ extern void diff_tree_combined_merge(const unsigned char *sha1, int, struct rev_
 
 void diff_set_mnemonic_prefix(struct diff_options *options, const char *a, const char *b);
 
+extern int diff_can_quit_early(struct diff_options *);
+
 extern void diff_addremove(struct diff_options *,
 			   int addremove,
 			   unsigned mode,