transport-helper: add 'force' to 'export' helpers

Otherwise they cannot know when to force the push or not (other than
hacks).

Tests-by: Richard Hansen <rhansen@bbn.com>
Documentation-by: Richard Hansen <rhansen@bbn.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitremote-helpers.txt

@@ -437,6 +437,10 @@ set by Git if the remote helper has the 'option' capability.
 'option check-connectivity' \{'true'|'false'\}::
 	Request the helper to check connectivity of a clone.
 
+'option force' \{'true'|'false'\}::
+	Request the helper to perform a force update.  Defaults to
+	'false'.
+
 SEE ALSO
 --------
 linkgit:git-remote[1]

git-remote-testgit.sh

@@ -15,6 +15,8 @@ test -z "$refspec" && prefix="refs"
 
 export GIT_DIR="$url/.git"
 
+force=
+
 mkdir -p "$dir"
 
 if test -z "$GIT_REMOTE_TESTGIT_NO_MARKS"
@@ -39,6 +41,7 @@ do
 		fi
 		test -n "$GIT_REMOTE_TESTGIT_SIGNED_TAGS" && echo "signed-tags"
 		test -n "$GIT_REMOTE_TESTGIT_NO_PRIVATE_UPDATE" && echo "no-private-update"
+		echo 'option'
 		echo
 		;;
 	list)
@@ -93,6 +96,7 @@ do
 		before=$(git for-each-ref --format=' %(refname) %(objectname) ')
 
 		git fast-import \
+			${force:+--force} \
 			${testgitmarks:+"--import-marks=$testgitmarks"} \
 			${testgitmarks:+"--export-marks=$testgitmarks"} \
 			--quiet
@@ -115,6 +119,20 @@ do
 
 		echo
 		;;
+	option\ *)
+		read cmd opt val <<-EOF
+		$line
+		EOF
+		case $opt in
+		force)
+			test $val = "true" && force="true" || force=
+			echo "ok"
+			;;
+		*)
+			echo "unsupported"
+			;;
+		esac
+		;;
 	'')
 		exit
 		;;

t/t5801-remote-helpers.sh

@@ -94,6 +94,19 @@ test_expect_failure 'push new branch with old:new refspec' '
 	compare_refs local HEAD server refs/heads/new-refspec
 '
 
+test_expect_success 'forced push' '
+	(cd local &&
+	git checkout -b force-test &&
+	echo content >> file &&
+	git commit -a -m eight &&
+	git push origin force-test &&
+	echo content >> file &&
+	git commit -a --amend -m eight-modified &&
+	git push --force origin force-test
+	) &&
+	compare_refs local refs/heads/force-test server refs/heads/force-test
+'
+
 test_expect_success 'cloning without refspec' '
 	GIT_REMOTE_TESTGIT_REFSPEC="" \
 	git clone "testgit::${PWD}/server" local2 2>error &&

transport-helper.c

@@ -854,6 +854,11 @@ static int push_refs_with_export(struct transport *transport,
 			die("helper %s does not support dry-run", data->name);
 	}
 
+	if (flags & TRANSPORT_PUSH_FORCE) {
+		if (set_helper_option(transport, "force", "true") != 0)
+			warning("helper %s does not support 'force'", data->name);
+	}
+
 	helper = get_helper(transport);
 
 	write_constant(helper->in, "export\n");