chunk-format.h: extract oid_version()

There are three definitions of an identical function which converts
`the_hash_algo` into either 1 (for SHA-1) or 2 (for SHA-256). There is a
copy of this function for writing both the commit-graph and
multi-pack-index file, and another inline definition used to write the
.rev header.

Consolidate these into a single definition in chunk-format.h. It's not
clear that this is the best header to define this function in, but it
should do for now.

(Worth noting, the .rev caller expects a 4-byte unsigned, but the other
two callers work with a single unsigned byte. The consolidated version
uses the latter type, and lets the compiler widen it when required).

Another caller will be added in a subsequent patch.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

chunk-format.h

@@ -2,6 +2,7 @@
 #define CHUNK_FORMAT_H
 
 #include "git-compat-util.h"
+#include "hash.h"
 
 struct hashfile;
 struct chunkfile;
@@ -65,4 +66,6 @@ int read_chunk(struct chunkfile *cf,
 	       chunk_read_fn fn,
 	       void *data);
 
+uint8_t oid_version(const struct git_hash_algo *algop);
+
 #endif