commit-graph: write Bloom filters to commit graph file

Update the technical documentation for commit-graph-format with
the formats for the Bloom filter index (BIDX) and Bloom filter
data (BDAT) chunks. Write the computed Bloom filters information
to the commit graph file using this format.

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>

Documentation/technical/commit-graph-format.txt

@@ -17,6 +17,9 @@ metadata, including:
 - The parents of the commit, stored using positional references within
   the graph file.
 
+- The Bloom filter of the commit carrying the paths that were changed between
+  the commit and its first parent, if requested.
+
 These positional references are stored as unsigned 32-bit integers
 corresponding to the array position within the list of commit OIDs. Due
 to some special constants we use to track parents, we can store at most
@@ -93,6 +96,33 @@ CHUNK DATA:
       positions for the parents until reaching a value with the most-significant
       bit on. The other bits correspond to the position of the last parent.
 
+  Bloom Filter Index (ID: {'B', 'I', 'D', 'X'}) (N * 4 bytes) [Optional]
+    * The ith entry, BIDX[i], stores the number of 8-byte word blocks in all
+      Bloom filters from commit 0 to commit i (inclusive) in lexicographic
+      order. The Bloom filter for the i-th commit spans from BIDX[i-1] to
+      BIDX[i] (plus header length), where BIDX[-1] is 0.
+    * The BIDX chunk is ignored if the BDAT chunk is not present.
+
+  Bloom Filter Data (ID: {'B', 'D', 'A', 'T'}) [Optional]
+    * It starts with header consisting of three unsigned 32-bit integers:
+      - Version of the hash algorithm being used. We currently only support
+	value 1 which corresponds to the 32-bit version of the murmur3 hash
+	implemented exactly as described in
+	https://en.wikipedia.org/wiki/MurmurHash#Algorithm and the double
+	hashing technique using seed values 0x293ae76f and 0x7e646e2 as
+	described in https://doi.org/10.1007/978-3-540-30494-4_26 "Bloom Filters
+	in Probabilistic Verification"
+      - The number of times a path is hashed and hence the number of bit positions
+	      that cumulatively determine whether a file is present in the commit.
+      - The minimum number of bits 'b' per entry in the Bloom filter. If the filter
+	      contains 'n' entries, then the filter size is the minimum number of 64-bit
+	      words that contain n*b bits.
+    * The rest of the chunk is the concatenation of all the computed Bloom
+      filters for the commits in lexicographic order.
+    * Note: Commits with no changes or more than 512 changes have Bloom filters
+      of length zero.
+    * The BDAT chunk is present if and only if BIDX is present.
+
   Base Graphs List (ID: {'B', 'A', 'S', 'E'}) [Optional]
       This list of H-byte hashes describe a set of B commit-graph files that
       form a commit-graph chain. The graph position for the ith commit in this