[PATCH 4/3] pack-bitmap.h: fix unused variable warning

[Denton Liu <liu.denton@gmail.com>]

When we ran `make hdr-check`, we got the following warning on Arch Linux:

	pack-bitmap.h:20:19: error: ‘BITMAP_IDX_SIGNATURE’ defined but not used [-Werror=unused-const-variable=]
	   20 | static const char BITMAP_IDX_SIGNATURE[] = {'B', 'I', 'T', 'M'};
	      |                   ^~~~~~~~~~~~~~~~~~~~
	cc1: all warnings being treated as errors

"Use" the BITMAP_IDX_SIGNATURE variable by making the size of
bitmap_disk_header.magic equal to the size of BITMAP_IDX_SIGNATURE. An
alternative was to simply add MAYBE_UNUSED. However, this design was
chosen because we eliminate the magic number (4) in the process.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
---
I'm tacking this patch on since this warning didn't show up until I
compiled it on gcc 9.1.0.

 pack-bitmap.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/pack-bitmap.h b/pack-bitmap.h
index 00de3ec8e4..466c5afa09 100644
--- a/pack-bitmap.h
+++ b/pack-bitmap.h
@@ -9,16 +9,16 @@ struct commit;
 struct repository;
 struct rev_info;
 
+static const char BITMAP_IDX_SIGNATURE[] = {'B', 'I', 'T', 'M'};
+
 struct bitmap_disk_header {
-	char magic[4];
+	char magic[ARRAY_SIZE(BITMAP_IDX_SIGNATURE)];
 	uint16_t version;
 	uint16_t options;
 	uint32_t entry_count;
 	unsigned char checksum[GIT_MAX_RAWSZ];
 };
 
-static const char BITMAP_IDX_SIGNATURE[] = {'B', 'I', 'T', 'M'};
-
 #define NEEDS_BITMAP (1u<<22)
 
 enum pack_bitmap_opts {
-- 
2.23.0.248.g3a9dd8fb08