commit: add for_each_mergetag()

In the same way as there is for_each_ref() to iterate on refs,
for_each_mergetag() allows the caller to iterate on the mergetags of
a given commit.  Use it to rewrite show_mergetag() used in "git log".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

diff --git a/commit.c b/commit.c
index 4ff8077..94b1af9 100644
--- a/commit.c
+++ b/commit.c
@@ -1316,6 +1316,19 @@
 	return extra;
 }
 
+void for_each_mergetag(each_mergetag_fn fn, struct commit *commit, void *data)
+{
+	struct commit_extra_header *extra, *to_free;
+
+	to_free = read_commit_extra_headers(commit, NULL);
+	for (extra = to_free; extra; extra = extra->next) {
+		if (strcmp(extra->key, "mergetag"))
+			continue; /* not a merge tag */
+		fn(commit, extra, data);
+	}
+	free_commit_extra_headers(to_free);
+}
+
 static inline int standard_header_field(const char *field, size_t len)
 {
 	return ((len == 4 && !memcmp(field, "tree ", 5)) ||

commit.h

diff --git a/commit.h b/commit.h
index 2e1492a..b695aa4 100644
--- a/commit.h
+++ b/commit.h
@@ -312,6 +312,11 @@
 
 extern void free_commit_extra_headers(struct commit_extra_header *extra);
 
+typedef void (*each_mergetag_fn)(struct commit *commit, struct commit_extra_header *extra,
+				 void *cb_data);
+
+extern void for_each_mergetag(each_mergetag_fn fn, struct commit *commit, void *data);
+
 struct merge_remote_desc {
 	struct object *obj; /* the named object, could be a tag */
 	const char *name;

log-tree.c

diff --git a/log-tree.c b/log-tree.c
index 10e6844..706ed4c 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -413,10 +413,11 @@
 		&& !commit->parents->next->next);
 }
 
-static void show_one_mergetag(struct rev_info *opt,
+static void show_one_mergetag(struct commit *commit,
 			      struct commit_extra_header *extra,
-			      struct commit *commit)
+			      void *data)
 {
+	struct rev_info *opt = (struct rev_info *)data;
 	unsigned char sha1[20];
 	struct tag *tag;
 	struct strbuf verify_message;
@@ -463,15 +464,7 @@
 
 static void show_mergetag(struct rev_info *opt, struct commit *commit)
 {
-	struct commit_extra_header *extra, *to_free;
-
-	to_free = read_commit_extra_headers(commit, NULL);
-	for (extra = to_free; extra; extra = extra->next) {
-		if (strcmp(extra->key, "mergetag"))
-			continue; /* not a merge tag */
-		show_one_mergetag(opt, extra, commit);
-	}
-	free_commit_extra_headers(to_free);
+	for_each_mergetag(show_one_mergetag, commit, opt);
 }
 
 void show_log(struct rev_info *opt)