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/t7001-mv.sh

@@ -177,7 +177,7 @@ test_expect_success "Sergey Vlasov's test case" '
 	date >ab.c &&
 	date >ab/d &&
 	git add ab.c ab &&
-	git commit -m 'initial' &&
+	git commit -m "initial" &&
 	git mv ab a
 '
 
@@ -248,6 +248,23 @@ test_expect_success 'git mv should not change sha1 of moved cache entry' '
 
 rm -f dirty dirty2
 
+# NB: This test is about the error message
+# as well as the failure.
+test_expect_success 'git mv error on conflicted file' '
+	rm -fr .git &&
+	git init &&
+	>conflict &&
+	test_when_finished "rm -f conflict" &&
+	cfhash=$(git hash-object -w conflict) &&
+	q_to_tab <<-EOF | git update-index --index-info &&
+	0 $cfhash 0Qconflict
+	100644 $cfhash 1Qconflict
+	EOF
+
+	test_must_fail git mv conflict newname 2>actual &&
+	test_i18ngrep "conflicted" actual
+'
+
 test_expect_success 'git mv should overwrite symlink to a file' '
 
 	rm -fr .git &&