merge-recursive: make the error-message generation an extern function

The construction of the struct unpack_trees_error_msgs was done within
git_merge_trees(), which prevented using the same messages easily from
another function.

[jc: backported for 1.6.5 maint before advice_commit_before_merge]

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge-recursive.h b/merge-recursive.h
index fd138ca..d8bc729 100644
--- a/merge-recursive.h
+++ b/merge-recursive.h
@@ -17,6 +17,9 @@
 	struct string_list current_directory_set;
 };
 
+/* Return a list of user-friendly error messages to be used by merge */
+struct unpack_trees_error_msgs get_porcelain_error_msgs(void);
+
 /* merge_trees() but with recursive ancestor consolidation */
 int merge_recursive(struct merge_options *o,
 		    struct commit *h1,