submodule: Search for merges only at end of recursive merge

The submodule merge search is not useful during virtual merges because
the results cannot be used automatically.  Furthermore any suggestions
made by the search may apply to commits different than HEAD:sub and
MERGE_HEAD:sub, thus confusing the user.  Skip searching for submodule
merges during a virtual merge such as that between B and C while merging
the heads of:

    B---BC
   / \ /
  A   X
   \ / \
    C---CB

Run the search only when the recursion level is zero (!o->call_depth).
This fixes known breakage tested in t7405-submodule-merge.

Signed-off-by: Brad King <brad.king@kitware.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

submodule.c

@@ -794,7 +794,7 @@ static void print_commit(struct commit *commit)
 
 int merge_submodule(unsigned char result[20], const char *path,
 		    const unsigned char base[20], const unsigned char a[20],
-		    const unsigned char b[20])
+		    const unsigned char b[20], int search)
 {
 	struct commit *commit_base, *commit_a, *commit_b;
 	int parent_count;
@@ -849,6 +849,10 @@ int merge_submodule(unsigned char result[20], const char *path,
 	 * user needs to confirm the resolution.
 	 */
 
+	/* Skip the search if makes no sense to the calling context.  */
+	if (!search)
+		return 0;
+
 	/* find commit which merges them */
 	parent_count = find_first_merges(&merges, path, commit_a, commit_b);
 	switch (parent_count) {