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: NJunio C Hamano <gitster@pobox.com>

diff-lib.c

@@ -73,9 +73,7 @@ int run_diff_files(struct rev_info *revs, unsigned int option)
 		struct cache_entry *ce = active_cache[i];
 		int changed;
 
-		if (DIFF_OPT_TST(&revs->diffopt, QUICK) &&
-		    !revs->diffopt.filter &&
-		    DIFF_OPT_TST(&revs->diffopt, HAS_CHANGES))
+		if (diff_can_quit_early(&revs->diffopt))
 			break;
 
 		if (!ce_path_match(ce, revs->prune_data))

diff.c

@@ -3513,6 +3513,13 @@ int diff_result_code(struct diff_options *opt, int status)
 	return result;
 }
 
+int diff_can_quit_early(struct diff_options *opt)
+{
+	return (DIFF_OPT_TST(opt, QUICK) &&
+		!opt->filter &&
+		DIFF_OPT_TST(opt, HAS_CHANGES));
+}
+
 void diff_addremove(struct diff_options *options,
 		    int addremove, unsigned mode,
 		    const unsigned char *sha1,

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,

tree-diff.c

@@ -286,9 +286,7 @@ int diff_tree(struct tree_desc *t1, struct tree_desc *t2, const char *base, stru
 	int baselen = strlen(base);
 
 	for (;;) {
-		if (DIFF_OPT_TST(opt, QUICK) &&
-		    !opt->filter &&
-		    DIFF_OPT_TST(opt, HAS_CHANGES))
+		if (diff_can_quit_early(opt))
 			break;
 		if (opt->nr_paths) {
 			skip_uninteresting(t1, base, baselen, opt);