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/templates/hooks--pre-push.sample

@@ -14,7 +14,7 @@
 # Information about the commits which are being pushed is supplied as lines to
 # the standard input in the form:
 #
-#   <local ref> <local sha1> <remote ref> <remote sha1>
+#   <local ref> <local oid> <remote ref> <remote oid>
 #
 # This sample shows how to prevent push of commits where the log message starts
 # with "WIP" (work in progress).
@@ -22,27 +22,27 @@
 remote="$1"
 url="$2"
 
-z40=0000000000000000000000000000000000000000
+zero=$(git hash-object --stdin </dev/null | tr '[0-9a-f]' '0')
 
-while read local_ref local_sha remote_ref remote_sha
+while read local_ref local_oid remote_ref remote_oid
 do
-	if [ "$local_sha" = $z40 ]
+	if test "$local_oid" = "$zero"
 	then
 		# Handle delete
 		:
 	else
-		if [ "$remote_sha" = $z40 ]
+		if test "$remote_oid" = "$zero"
 		then
 			# New branch, examine all commits
-			range="$local_sha"
+			range="$local_oid"
 		else
 			# Update to existing branch, examine new commits
-			range="$remote_sha..$local_sha"
+			range="$remote_oid..$local_oid"
 		fi
 
 		# Check for WIP commit
-		commit=`git rev-list -n 1 --grep '^WIP' "$range"`
-		if [ -n "$commit" ]
+		commit=$(git rev-list -n 1 --grep '^WIP' "$range")
+		if test -n "$commit"
 		then
 			echo >&2 "Found WIP commit in $local_ref, not pushing"
 			exit 1