update-index --refresh --porcelain: add missing const

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/cache.h b/cache.h
index 3abf895..4a758ba 100644
--- a/cache.h
+++ b/cache.h
@@ -511,7 +511,7 @@
 #define REFRESH_IGNORE_MISSING	0x0008	/* ignore non-existent */
 #define REFRESH_IGNORE_SUBMODULES	0x0010	/* ignore submodules */
 #define REFRESH_IN_PORCELAIN	0x0020	/* user friendly output, not "needs update" */
-extern int refresh_index(struct index_state *, unsigned int flags, const char **pathspec, char *seen, char *header_msg);
+extern int refresh_index(struct index_state *, unsigned int flags, const char **pathspec, char *seen, const char *header_msg);
 
 struct lock_file {
 	struct lock_file *next;