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/t7105-reset-patch.sh

@@ -38,6 +38,27 @@ test_expect_success PERL 'git reset -p HEAD^' '
 	test_i18ngrep "Apply" output
 '
 
+test_expect_success PERL 'git reset -p HEAD^^{tree}' '
+	test_write_lines n y | git reset -p HEAD^^{tree} >output &&
+	verify_state dir/foo work parent &&
+	verify_saved_state bar &&
+	test_i18ngrep "Apply" output
+'
+
+test_expect_success PERL 'git reset -p HEAD^:dir/foo (blob fails)' '
+	set_and_save_state dir/foo work work &&
+	test_must_fail git reset -p HEAD^:dir/foo &&
+	verify_saved_state dir/foo &&
+	verify_saved_state bar
+'
+
+test_expect_success PERL 'git reset -p aaaaaaaa (unknown fails)' '
+	set_and_save_state dir/foo work work &&
+	test_must_fail git reset -p aaaaaaaa &&
+	verify_saved_state dir/foo &&
+	verify_saved_state bar
+'
+
 # The idea in the rest is that bar sorts first, so we always say 'y'
 # first and if the path limiter fails it'll apply to bar instead of
 # dir/foo.  There's always an extra 'n' to reject edits to dir/foo in