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

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

third_party/git/Documentation/git-index-pack.txt

@@ -93,11 +93,21 @@ OPTIONS
 --max-input-size=<size>::
 	Die, if the pack is larger than <size>.
 
+--object-format=<hash-algorithm>::
+	Specify the given object format (hash algorithm) for the pack.  The valid
+	values are 'sha1' and (if enabled) 'sha256'.  The default is the algorithm for
+	the current repository (set by `extensions.objectFormat`), or 'sha1' if no
+	value is set or outside a repository.
++
+This option cannot be used with --stdin.
++
+include::object-format-disclaimer.txt[]
+
 NOTES
 -----
 
-Once the index has been created, the list of object names is sorted
-and the SHA-1 hash of that list is printed to stdout. If --stdin was
+Once the index has been created, the hash that goes into the name of
+the pack/idx file is printed to stdout. If --stdin was
 also used then this is prefixed by either "pack\t", or "keep\t" if a
 new .keep file was successfully created. This is useful to remove a
 .keep file used as a lock to prevent the race with 'git repack'