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/config/push.txt

@@ -1,6 +1,7 @@
 push.default::
 	Defines the action `git push` should take if no refspec is
-	explicitly given.  Different values are well-suited for
+	given (whether from the command-line, config, or elsewhere).
+	Different values are well-suited for
 	specific workflows; for instance, in a purely central workflow
 	(i.e. the fetch source is equal to the push destination),
 	`upstream` is probably what you want.  Possible values are:
@@ -8,7 +9,7 @@ push.default::
 --
 
 * `nothing` - do not push anything (error out) unless a refspec is
-  explicitly given. This is primarily meant for people who want to
+  given. This is primarily meant for people who want to
   avoid mistakes by always being explicit.
 
 * `current` - push the current branch to update a branch with the same
@@ -79,7 +80,7 @@ higher priority configuration file (e.g. `.git/config` in a
 repository) to clear the values inherited from a lower priority
 configuration files (e.g. `$HOME/.gitconfig`).
 +
---
+----
 
 Example:
 
@@ -96,7 +97,7 @@ repo/.git/config
 
 This will result in only b (a and c are cleared).
 
---
+----
 
 push.recurseSubmodules::
 	Make sure all submodule commits used by the revisions to be pushed
@@ -111,3 +112,5 @@ push.recurseSubmodules::
 	is 'no' then default behavior of ignoring submodules when pushing
 	is retained. You may override this configuration at time of push by
 	specifying '--recurse-submodules=check|on-demand|no'.
+	If not set, 'no' is used by default, unless 'submodule.recurse' is
+	set (in which case a 'true' value means 'on-demand').