pack-write: pass hash_algo to write_rev_file()

The `write_rev_file()` function uses the global `the_hash_algo` variable
to access the repository's hash_algo. To avoid global variable usage,
pass a hash_algo from the layers above. Also modify children functions
`write_rev_file_order()` and `write_rev_header()` to accept
'the_hash_algo'.

Altough the layers above could have access to the hash_algo internally,
simply pass in `the_hash_algo`. This avoids any compatibility issues and
bubbles up global variable usage to upper layers which can be eventually
resolved.

However, in `midx-write.c`, since all usage of global variables is
removed, don't reintroduce them and instead use the `repo` available in
the context.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pack.h

@@ -105,8 +105,18 @@ struct ref;
 
 void write_promisor_file(const char *promisor_name, struct ref **sought, int nr_sought);
 
-char *write_rev_file(const char *rev_name, struct pack_idx_entry **objects, uint32_t nr_objects, const unsigned char *hash, unsigned flags);
-char *write_rev_file_order(const char *rev_name, uint32_t *pack_order, uint32_t nr_objects, const unsigned char *hash, unsigned flags);
+char *write_rev_file(const struct git_hash_algo *hash_algo,
+		     const char *rev_name,
+		     struct pack_idx_entry **objects,
+		     uint32_t nr_objects,
+		     const unsigned char *hash,
+		     unsigned flags);
+char *write_rev_file_order(const struct git_hash_algo *hash_algo,
+			   const char *rev_name,
+			   uint32_t *pack_order,
+			   uint32_t nr_objects,
+			   const unsigned char *hash,
+			   unsigned flags);
 
 /*
  * The "hdr" output buffer should be at least this big, which will handle sizes