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/gitsubmodules.txt

@@ -3,7 +3,7 @@ gitsubmodules(7)
 
 NAME
 ----
-gitsubmodules - mounting one repository inside another
+gitsubmodules - Mounting one repository inside another
 
 SYNOPSIS
 --------
@@ -225,10 +225,10 @@ presence of the .url field.
 Workflow for a third party library
 ----------------------------------
 
-  # add a submodule
+  # Add a submodule
   git submodule add <url> <path>
 
-  # occasionally update the submodule to a new version:
+  # Occasionally update the submodule to a new version:
   git -C <path> checkout <new version>
   git add <path>
   git commit -m "update submodule to new version"
@@ -246,20 +246,23 @@ Workflow for an artificially split repo
   # regular commands recurse into submodules by default
   git config --global submodule.recurse true
 
-  # Unlike the other commands below clone still needs
+  # Unlike most other commands below, clone still needs
   # its own recurse flag:
   git clone --recurse <URL> <directory>
   cd <directory>
 
   # Get to know the code:
   git grep foo
-  git ls-files
+  git ls-files --recurse-submodules
+
+[NOTE]
+`git ls-files` also requires its own `--recurse-submodules` flag.
 
   # Get new code
   git fetch
   git pull --rebase
 
-  # change worktree
+  # Change worktree
   git checkout
   git reset
 
@@ -267,11 +270,12 @@ Implementation details
 ----------------------
 
 When cloning or pulling a repository containing submodules the submodules
-will not be checked out by default; You can instruct 'clone' to recurse
-into submodules. The 'init' and 'update' subcommands of 'git submodule'
+will not be checked out by default; you can instruct `clone` to recurse
+into submodules. The `init` and `update` subcommands of `git submodule`
 will maintain submodules checked out and at an appropriate revision in
-your working tree. Alternatively you can set 'submodule.recurse' to have
-'checkout' recursing into submodules.
+your working tree. Alternatively you can set `submodule.recurse` to have
+`checkout` recursing into submodules (note that `submodule.recurse` also
+affects other Git commands, see linkgit:git-config[1] for a complete list).
 
 
 SEE ALSO