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-clean.txt

@@ -26,18 +26,20 @@ are affected.
 OPTIONS
 -------
 -d::
-	Remove untracked directories in addition to untracked files.
-	If an untracked directory is managed by a different Git
-	repository, it is not removed by default.  Use -f option twice
-	if you really want to remove such a directory.
+	Normally, when no <path> is specified, git clean will not
+	recurse into untracked directories to avoid removing too much.
+	Specify -d to have it recurse into such directories as well.
+	If any paths are specified, -d is irrelevant; all untracked
+	files matching the specified paths (with exceptions for nested
+	git directories mentioned under `--force`) will be removed.
 
 -f::
 --force::
 	If the Git configuration variable clean.requireForce is not set
 	to false, 'git clean' will refuse to delete files or directories
-	unless given -f, -n or -i. Git will refuse to delete directories
-	with .git sub directory or file unless a second -f
-	is given.
+	unless given -f or -i.  Git will refuse to modify untracked
+	nested git repositories (directories with a .git subdirectory)
+	unless a second -f is given.
 
 -i::
 --interactive::