diff: generate pretty filenames in prep_temp_blob()

Naturally, prep_temp_blob() did not care about filenames.
As a result, GIT_EXTERNAL_DIFF and textconv generated
filenames such as ".diff_XXXXXX".

This modifies prep_temp_blob() to generate user-friendly
filenames when creating temporary files.

Diffing "name.ext" now generates "XXXXXX_name.ext".

Signed-off-by: David Aguilar <davvid@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -614,6 +614,8 @@ extern int is_empty_blob_sha1(const unsigned char *sha1);
 
 int git_mkstemp(char *path, size_t n, const char *template);
 
+int git_mkstemps(char *path, size_t n, const char *template, int suffix_len);
+
 /*
  * NOTE NOTE NOTE!!
  *