binary diff: further updates.

This updates the user interface and generated diff data format.

 * "diff --binary" is used to signal that we want an e-mailable
   binary patch.  It implies --full-index and -p.

 * "apply --allow-binary-replacement" acquired a short synonym
   "apply --binary".

 * After the "GIT binary patch\n" header line there is a token
   to record which binary patch mechanism was used, so that we
   can extend it later.  Currently there are two mechanisms
   defined: "literal" and "delta".  The former records the
   deflated postimage and the latter records the deflated delta
   from the preimage to postimage.

   For purely implementation convenience, I added the deflated
   length after these "literal/delta" tokens (otherwise the
   decoding side needs to guess and reallocate the buffer while
   inflating).  Improvement patches are very welcomed.

Signed-off-by: Junio C Hamano <junkio@cox.net>

cache.h

@@ -365,5 +365,6 @@ extern void setup_pager(void);
 
 /* base85 */
 int decode_85(char *dst, char *line, int linelen);
+void encode_85(char *buf, unsigned char *data, int bytes);
 
 #endif /* CACHE_H */