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/git-gui/lib/branch.tcl

@@ -8,6 +8,7 @@ proc load_all_heads {} {
 	set rh_len [expr {[string length $rh] + 1}]
 	set all_heads [list]
 	set fd [git_read for-each-ref --format=%(refname) $rh]
+	fconfigure $fd -translation binary -encoding utf-8
 	while {[gets $fd line] > 0} {
 		if {!$some_heads_tracking || ![is_tracking_branch $line]} {
 			lappend all_heads [string range $line $rh_len end]
@@ -24,6 +25,7 @@ proc load_all_tags {} {
 		--sort=-taggerdate \
 		--format=%(refname) \
 		refs/tags]
+	fconfigure $fd -translation binary -encoding utf-8
 	while {[gets $fd line] > 0} {
 		if {![regsub ^refs/tags/ $line {} name]} continue
 		lappend all_tags $name