commit-graph: reuse existing Bloom filters during write

Add logic to
a) parse Bloom filter information from the commit graph file and,
b) re-use existing Bloom filters.

See Documentation/technical/commit-graph-format for the format in which
the Bloom filter information is written to the commit graph file.

To read Bloom filter for a given commit with lexicographic position
'i' we need to:
1. Read BIDX[i] which essentially gives us the starting index in BDAT for
   filter of commit i+1. It is essentially the index past the end
   of the filter of commit i. It is called end_index in the code.

2. For i>0, read BIDX[i-1] which will give us the starting index in BDAT
   for filter of commit i. It is called the start_index in the code.
   For the first commit, where i = 0, Bloom filter data starts at the
   beginning, just past the header in the BDAT chunk. Hence, start_index
   will be 0.

3. The length of the filter will be end_index - start_index, because
   BIDX[i] gives the cumulative 8-byte words including the ith
   commit's filter.

We toggle whether Bloom filters should be recomputed based on the
compute_if_not_present flag.

Helped-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Garima Singh <garima.singh@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bloom.h

@@ -32,6 +32,7 @@ struct bloom_filter_settings {
 
 #define DEFAULT_BLOOM_FILTER_SETTINGS { 1, 7, 10 }
 #define BITS_PER_WORD 8
+#define BLOOMDATA_CHUNK_HEADER_SIZE 3 * sizeof(uint32_t)
 
 /*
  * A bloom_filter struct represents a data segment to
@@ -79,6 +80,7 @@ void add_key_to_filter(const struct bloom_key *key,
 void init_bloom_filters(void);
 
 struct bloom_filter *get_bloom_filter(struct repository *r,
-				      struct commit *c);
+				      struct commit *c,
+				      int compute_if_not_present);
 
 #endif