merge(3p/git): Merge git subtree at v2.29.2 r/1890

This also bumps the stable nixpkgs to 20.09 as of 2020-11-21, because
there is some breakage in the git build related to the netrc
credentials helper which someone has taken care of in nixpkgs.

The stable channel is not used for anything other than git, so this
should be fine.

Change-Id: I3575a19dab09e1e9556cf8231d717de9890484fb

1 files changed, 30 insertions, 18 deletions

diff --git a/third_party/git/Documentation/technical/pack-format.txt b/third_party/git/Documentation/technical/pack-format.txt
index cab5bdd2ff0f..f96b2e605f34 100644
--- a/third_party/git/Documentation/technical/pack-format.txt
+++ b/third_party/git/Documentation/technical/pack-format.txt
@@ -1,6 +1,12 @@
 Git pack format
 ===============
 
+== Checksums and object IDs
+
+In a repository using the traditional SHA-1, pack checksums, index checksums,
+and object IDs (object names) mentioned below are all computed using SHA-1.
+Similarly, in SHA-256 repositories, these values are computed using SHA-256.
+
 == pack-*.pack files have the following format:
 
    - A header appears at the beginning and consists of the following:
@@ -26,7 +32,7 @@ Git pack format
 
      (deltified representation)
      n-byte type and length (3-bit type, (n-1)*7+4-bit length)
-     20-byte base object name if OBJ_REF_DELTA or a negative relative
+     base object name if OBJ_REF_DELTA or a negative relative
 	 offset from the delta object's position in the pack if this
 	 is an OBJ_OFS_DELTA object
      compressed delta data
@@ -34,7 +40,7 @@ Git pack format
      Observation: length of each object is encoded in a variable
      length format and is not constrained to 32-bit or anything.
 
-  - The trailer records 20-byte SHA-1 checksum of all of the above.
+  - The trailer records a pack checksum of all of the above.
 
 === Object types
 
@@ -58,8 +64,8 @@ ofs-delta and ref-delta, which is only valid in a pack file.
 
 Both ofs-delta and ref-delta store the "delta" to be applied to
 another object (called 'base object') to reconstruct the object. The
-difference between them is, ref-delta directly encodes 20-byte base
-object name. If the base object is in the same pack, ofs-delta encodes
+difference between them is, ref-delta directly encodes base object
+name. If the base object is in the same pack, ofs-delta encodes
 the offset of the base object in the pack instead.
 
 The base object could also be deltified if it's in the same pack.
@@ -143,14 +149,14 @@ This is the instruction reserved for future expansion.
     object is stored in the packfile as the offset from the
     beginning.
 
-    20-byte object name.
+    one object name of the appropriate size.
 
   - The file is concluded with a trailer:
 
-    A copy of the 20-byte SHA-1 checksum at the end of
-    corresponding packfile.
+    A copy of the pack checksum at the end of the corresponding
+    packfile.
 
-    20-byte SHA-1-checksum of all of the above.
+    Index checksum of all of the above.
 
 Pack Idx file:
 
@@ -198,7 +204,7 @@ Pack file entry: <+
         If it is not DELTA, then deflated bytes (the size above
 		is the size before compression).
 	If it is REF_DELTA, then
-	  20-byte base object name SHA-1 (the size above is the
+	  base object name (the size above is the
 		size of the delta data that follows).
           delta data, deflated.
 	If it is OFS_DELTA, then
@@ -227,9 +233,9 @@ Pack file entry: <+
 
   - A 256-entry fan-out table just like v1.
 
-  - A table of sorted 20-byte SHA-1 object names.  These are
-    packed together without offset values to reduce the cache
-    footprint of the binary search for a specific object name.
+  - A table of sorted object names.  These are packed together
+    without offset values to reduce the cache footprint of the
+    binary search for a specific object name.
 
   - A table of 4-byte CRC32 values of the packed object data.
     This is new in v2 so compressed data can be copied directly
@@ -248,10 +254,10 @@ Pack file entry: <+
 
   - The same trailer as a v1 pack file:
 
-    A copy of the 20-byte SHA-1 checksum at the end of
+    A copy of the pack checksum at the end of
     corresponding packfile.
 
-    20-byte SHA-1-checksum of all of the above.
+    Index checksum of all of the above.
 
 == multi-pack-index (MIDX) files have the following format:
 
@@ -273,7 +279,12 @@ HEADER:
 	    Git only writes or recognizes version 1.
 
 	1-byte Object Id Version
-	    Git only writes or recognizes version 1 (SHA1).
+	    We infer the length of object IDs (OIDs) from this value:
+		1 => SHA-1
+		2 => SHA-256
+	    If the hash type does not match the repository's hash algorithm,
+	    the multi-pack-index file should be ignored with a warning
+	    presented to the user.
 
 	1-byte number of "chunks"
 
@@ -315,10 +326,11 @@ CHUNK DATA:
 	    Stores two 4-byte values for every object.
 	    1: The pack-int-id for the pack storing this object.
 	    2: The offset within the pack.
-		If all offsets are less than 2^31, then the large offset chunk
+		If all offsets are less than 2^32, then the large offset chunk
 		will not exist and offsets are stored as in IDX v1.
 		If there is at least one offset value larger than 2^32-1, then
-		the large offset chunk must exist. If the large offset chunk
+		the large offset chunk must exist, and offsets larger than
+		2^31-1 must be stored in it instead. If the large offset chunk
 		exists and the 31st bit is on, then removing that bit reveals
 		the row in the large offsets containing the 8-byte offset of
 		this object.
@@ -328,4 +340,4 @@ CHUNK DATA:
 
 TRAILER:
 
-	20-byte SHA1-checksum of the above contents.
+	Index checksum of the above contents.