[PATCH] Constness fix for pickaxe option.

Constness fix for pickaxe option.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/diff-files.c b/diff-files.c
index a3c523f..6dca9e0 100644
--- a/diff-files.c
+++ b/diff-files.c
@@ -14,7 +14,7 @@
 static int detect_rename = 0;
 static int reverse_diff = 0;
 static int diff_score_opt = 0;
-static char *pickaxe = 0;
+static const char *pickaxe = NULL;
 static int silent = 0;
 
 static int matches_pathspec(struct cache_entry *ce, char **spec, int cnt)