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-init.txt

@@ -10,7 +10,8 @@ SYNOPSIS
 --------
 [verse]
 'git init' [-q | --quiet] [--bare] [--template=<template_directory>]
-	  [--separate-git-dir <git dir>]
+	  [--separate-git-dir <git dir>] [--object-format=<format>]
+	  [-b <branch-name> | --initial-branch=<branch-name>]
 	  [--shared[=<permissions>]] [directory]
 
 
@@ -48,6 +49,13 @@ Only print error and warning messages; all other output will be suppressed.
 Create a bare repository. If `GIT_DIR` environment is not set, it is set to the
 current working directory.
 
+--object-format=<format>::
+
+Specify the given object format (hash algorithm) for the repository.  The valid
+values are 'sha1' and (if enabled) 'sha256'.  'sha1' is the default.
++
+include::object-format-disclaimer.txt[]
+
 --template=<template_directory>::
 
 Specify the directory from which templates will be used.  (See the "TEMPLATE
@@ -62,6 +70,12 @@ repository.
 +
 If this is reinitialization, the repository will be moved to the specified path.
 
+-b <branch-name>::
+--initial-branch=<branch-name>::
+
+Use the specified name for the initial branch in the newly created repository.
+If not specified, fall back to the default name: `master`.
+
 --shared[=(false|true|umask|group|all|world|everybody|0xxx)]::
 
 Specify that the Git repository is to be shared amongst several users.  This