midx: mark get_midx_checksum() arguments as const

To make clear that the fucntion `get_midx_checksum()` does not do
anything to modify its argument, mark the MIDX pointer as const.

The following commit will rename this function altogether to make clear
that it returns the raw bytes of the checksum, not a hex-encoded copy of
it.

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

Author: ttaylorr

midx.c

@@ -24,7 +24,7 @@ void clear_incremental_midx_files_ext(struct odb_source *source, const char *ext
 int cmp_idx_or_pack_name(const char *idx_or_pack_name,
 			 const char *idx_name);
 
-const unsigned char *get_midx_checksum(struct multi_pack_index *m)
+const unsigned char *get_midx_checksum(const struct multi_pack_index *m)
 {
 	return m->data + m->data_len - m->source->odb->repo->hash_algo->rawsz;
 }

midx.h

@@ -85,7 +85,7 @@ struct multi_pack_index {
 #define MIDX_EXT_BITMAP "bitmap"
 #define MIDX_EXT_MIDX "midx"
 
-const unsigned char *get_midx_checksum(struct multi_pack_index *m);
+const unsigned char *get_midx_checksum(const struct multi_pack_index *m);
 void get_midx_filename(struct odb_source *source, struct strbuf *out);
 void get_midx_filename_ext(struct odb_source *source, struct strbuf *out,
 			   const unsigned char *hash, const char *ext);