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

This variable is not used by any other file.

Signed-off-by: Nanako Shiraishi <nanako3@lavabit.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin-describe.c b/builtin-describe.c
index e515f9c..62a1905 100644
--- a/builtin-describe.c
+++ b/builtin-describe.c
@@ -20,7 +20,7 @@
 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 {