built-in add -p: handle diff.algorithm

The Perl version of `git add -p` reads the config setting
`diff.algorithm` and if set, uses it to generate the diff using the
specified algorithm.

This patch ports that functionality to the C version.

Note: just like `git-add--interactive.perl`, we do _not_ respect this
config setting in `git add -i`'s `diff` command, but _only_ in the
`patch` command.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

add-interactive.c

@@ -56,11 +56,16 @@ void init_add_i_state(struct add_i_state *s, struct repository *r)
 	FREE_AND_NULL(s->interactive_diff_filter);
 	git_config_get_string("interactive.difffilter",
 			      &s->interactive_diff_filter);
+
+	FREE_AND_NULL(s->interactive_diff_algorithm);
+	git_config_get_string("diff.algorithm",
+			      &s->interactive_diff_algorithm);
 }
 
 void clear_add_i_state(struct add_i_state *s)
 {
 	FREE_AND_NULL(s->interactive_diff_filter);
+	FREE_AND_NULL(s->interactive_diff_algorithm);
 	memset(s, 0, sizeof(*s));
 	s->use_color = -1;
 }

add-interactive.h

@@ -16,7 +16,7 @@ struct add_i_state {
 	char file_old_color[COLOR_MAXLEN];
 	char file_new_color[COLOR_MAXLEN];
 
-	char *interactive_diff_filter;
+	char *interactive_diff_filter, *interactive_diff_algorithm;
 };
 
 void init_add_i_state(struct add_i_state *s, struct repository *r);

add-patch.c

@@ -360,6 +360,7 @@ static int is_octal(const char *p, size_t len)
 static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 {
 	struct argv_array args = ARGV_ARRAY_INIT;
+	const char *diff_algorithm = s->s.interactive_diff_algorithm;
 	struct strbuf *plain = &s->plain, *colored = NULL;
 	struct child_process cp = CHILD_PROCESS_INIT;
 	char *p, *pend, *colored_p = NULL, *colored_pend = NULL, marker = '\0';
@@ -369,6 +370,8 @@ static int parse_diff(struct add_p_state *s, const struct pathspec *ps)
 	int res;
 
 	argv_array_pushv(&args, s->mode->diff_cmd);
+	if (diff_algorithm)
+		argv_array_pushf(&args, "--diff-algorithm=%s", diff_algorithm);
 	if (s->revision) {
 		struct object_id oid;
 		argv_array_push(&args,