sha1_file.c: make pack-name helper globally accessible

We provide sha1_pack_name() and sha1_pack_index_name(), but
the more generic form (which takes its own strbuf and an
arbitrary extension) is only used to implement the other
two.  Let's make it available, but clean up a few things:

  1. Name it odb_pack_name(), as the original
     sha1_get_pack_name() is long but not all that
     descriptive.

  2. Switch the strbuf argument to the beginning, so that it
     matches similar path-building functions like
     git_path_buf().

  3. Clean up the out-dated docstring and move it to the
     public declaration.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1568,6 +1568,15 @@ extern void pack_report(void);
  */
 extern int odb_mkstemp(char *template, size_t limit, const char *pattern);
 
+/*
+ * Generate the filename to be used for a pack file with checksum "sha1" and
+ * extension "ext". The result is written into the strbuf "buf", overwriting
+ * any existing contents. A pointer to buf->buf is returned as a convenience.
+ *
+ * Example: odb_pack_name(out, sha1, "idx") => ".git/objects/pack/pack-1234..idx"
+ */
+extern char *odb_pack_name(struct strbuf *buf, const unsigned char *sha1, const char *ext);
+
 /*
  * Create a pack .keep file in the object database's pack directory, for
  * a pack with checksum "sha1". The return value is a file descriptor opened