use git_path_* helper functions

Long ago we added functions like git_path_merge_msg() to
replace the more dangerous git_path("MERGE_MSG"). Over time
some new calls to the latter have crept it. Let's convert
them to use the safer form.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -821,9 +821,9 @@ static int prepare_to_commit(const char *index_file, const char *prefix,
 					"If this is not correct, please remove the file\n"
 					"	%s\n"
 					"and try again.\n"),
-				git_path(whence == FROM_MERGE
-					 ? "MERGE_HEAD"
-					 : "CHERRY_PICK_HEAD"));
+				whence == FROM_MERGE ?
+					git_path_merge_head() :
+					git_path_cherry_pick_head());
 		}
 
 		fprintf(s->fp, "\n");