builtin-describe.c: make a global variable "pattern" static

This variable is not used by any other file.
Signed-off-by: NNanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin-describe.c

@@ -20,7 +20,7 @@ static int tags;	/* But allow any tags if --tags is specified */
 static int longformat;
 static int abbrev = DEFAULT_ABBREV;
 static int max_candidates = 10;
-const char *pattern = NULL;
+static const char *pattern;
 static int always;
 
 struct commit_name {