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/t/t7506-status-submodule.sh

@@ -22,6 +22,10 @@ sanitize_output () {
 	mv output2 output
 }
 
+sanitize_diff () {
+	sed -e "/^index [0-9a-f,]*\.\.[0-9a-f]*/d" "$1"
+}
+
 
 test_expect_success 'setup' '
 	test_create_repo_with_commit sub &&
@@ -269,7 +273,6 @@ short_sha1_merge_sub1=$(cd sub1 && git rev-parse --short HEAD)
 short_sha1_merge_sub2=$(cd sub2 && git rev-parse --short HEAD)
 cat >diff_expect <<\EOF
 diff --cc .gitmodules
-index badaa4c,44f999a..0000000
 --- a/.gitmodules
 +++ b/.gitmodules
 @@@ -1,3 -1,3 +1,9 @@@
@@ -286,7 +289,6 @@ EOF
 
 cat >diff_submodule_expect <<\EOF
 diff --cc .gitmodules
-index badaa4c,44f999a..0000000
 --- a/.gitmodules
 +++ b/.gitmodules
 @@@ -1,3 -1,3 +1,9 @@@
@@ -306,7 +308,8 @@ test_expect_success 'diff with merge conflict in .gitmodules' '
 		cd super &&
 		git diff >../diff_actual 2>&1
 	) &&
-	test_cmp diff_expect diff_actual
+	sanitize_diff diff_actual >diff_sanitized &&
+	test_cmp diff_expect diff_sanitized
 '
 
 test_expect_success 'diff --submodule with merge conflict in .gitmodules' '
@@ -314,7 +317,8 @@ test_expect_success 'diff --submodule with merge conflict in .gitmodules' '
 		cd super &&
 		git diff --submodule >../diff_submodule_actual 2>&1
 	) &&
-	test_cmp diff_submodule_expect diff_submodule_actual
+	sanitize_diff diff_submodule_actual >diff_sanitized &&
+	test_cmp diff_submodule_expect diff_sanitized
 '
 
 # We'll setup different cases for further testing: