Move set_porcelain_error_msgs to unpack-trees.c and rename it

The function is currently dealing only with error messages, but the
intent of calling it is really to notify the unpack-tree mechanics that
it is running in porcelain mode.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

unpack-trees.h

@@ -22,6 +22,12 @@ enum unpack_trees_error_types {
 	NB_UNPACK_TREES_ERROR_TYPES
 };
 
+/*
+ * Sets the list of user-friendly error messages to be used by the
+ * command "cmd" (either merge or checkout)
+ */
+void setup_unpack_trees_porcelain(const char **msgs, const char *cmd);
+
 struct rejected_paths_list {
 	char *path;
 	struct rejected_paths_list *next;