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/INSTALL

@@ -109,15 +109,15 @@ Issues of note:
 
  - Git is reasonably self-sufficient, but does depend on a few external
    programs and libraries.  Git can be used without most of them by adding
-   the approriate "NO_<LIBRARY>=YesPlease" to the make command line or
+   the appropriate "NO_<LIBRARY>=YesPlease" to the make command line or
    config.mak file.
 
 	- "zlib", the compression library. Git won't build without it.
 
 	- "ssh" is used to push and pull over the net.
 
-	- A POSIX-compliant shell is required to run many scripts needed
-	  for everyday use (e.g. "bisect", "pull").
+	- A POSIX-compliant shell is required to run some scripts needed
+	  for everyday use (e.g. "bisect", "request-pull").
 
 	- "Perl" version 5.8 or later is needed to use some of the
 	  features (e.g. preparing a partial commit using "git add -i/-p",
@@ -206,9 +206,7 @@ Issues of note:
    clone two separate git-htmldocs and git-manpages repositories next
    to the clone of git itself.
 
-   It has been reported that docbook-xsl version 1.72 and 1.73 are
-   buggy; 1.72 misformats manual pages for callouts, and 1.73 needs
-   the patch in contrib/patches/docbook-xsl-manpages-charmap.patch
+   The minimum supported version of docbook-xsl is 1.74.
 
    Users attempting to build the documentation on Cygwin may need to ensure
    that the /etc/xml/catalog file looks something like this: