Make write_sha1_file_prepare() static

There are no callers of write_sha1_file_prepare() left outside of
sha1_file.c, so make it static.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

cache.h

@@ -247,12 +247,6 @@ extern void * unpack_sha1_file(void *map, unsigned long mapsize, char *type, uns
 extern void * read_sha1_file(const unsigned char *sha1, char *type, unsigned long *size);
 extern int hash_sha1_file(void *buf, unsigned long len, const char *type, unsigned char *sha1);
 extern int write_sha1_file(void *buf, unsigned long len, const char *type, unsigned char *return_sha1);
-extern char *write_sha1_file_prepare(void *buf,
-				     unsigned long len,
-				     const char *type,
-				     unsigned char *sha1,
-				     unsigned char *hdr,
-				     int *hdrlen);
 
 extern int check_sha1_signature(const unsigned char *sha1, void *buf, unsigned long size, const char *type);
 

sha1_file.c

@@ -1347,12 +1347,9 @@ void *read_object_with_reference(const unsigned char *sha1,
 	}
 }
 
-char *write_sha1_file_prepare(void *buf,
-			      unsigned long len,
-			      const char *type,
-			      unsigned char *sha1,
-			      unsigned char *hdr,
-			      int *hdrlen)
+static char *write_sha1_file_prepare(void *buf, unsigned long len,
+                                     const char *type, unsigned char *sha1,
+                                     unsigned char *hdr, int *hdrlen)
 {
 	SHA_CTX c;