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-add.txt

@@ -11,7 +11,8 @@ SYNOPSIS
 'git add' [--verbose | -v] [--dry-run | -n] [--force | -f] [--interactive | -i] [--patch | -p]
 	  [--edit | -e] [--[no-]all | --[no-]ignore-removal | [--update | -u]]
 	  [--intent-to-add | -N] [--refresh] [--ignore-errors] [--ignore-missing] [--renormalize]
-	  [--chmod=(+|-)x] [--] [<pathspec>...]
+	  [--chmod=(+|-)x] [--pathspec-from-file=<file> [--pathspec-file-nul]]
+	  [--] [<pathspec>...]
 
 DESCRIPTION
 -----------
@@ -187,6 +188,19 @@ for "git add --no-all <pathspec>...", i.e. ignored removed files.
 	bit is only changed in the index, the files on disk are left
 	unchanged.
 
+--pathspec-from-file=<file>::
+	Pathspec is passed in `<file>` instead of commandline args. If
+	`<file>` is exactly `-` then standard input is used. Pathspec
+	elements are separated by LF or CR/LF. Pathspec elements can be
+	quoted as explained for the configuration variable `core.quotePath`
+	(see linkgit:git-config[1]). See also `--pathspec-file-nul` and
+	global `--literal-pathspecs`.
+
+--pathspec-file-nul::
+	Only meaningful with `--pathspec-from-file`. Pathspec elements are
+	separated with NUL character and all other characters are taken
+	literally (including newlines and quotes).
+
 \--::
 	This option can be used to separate command-line options from
 	the list of files, (useful when filenames might be mistaken