core: Add --link-cache option to checkout

This is a convenient way to have a lookaside directory of hard links,
which can greatly speed up checkouts.  In the future we probably want
to push this down into the repository.

tests/t0000-basic.sh

@@ -19,7 +19,7 @@
 
 set -e
 
-echo "1..29"
+echo "1..30"
 
 . libtest.sh
 
@@ -211,3 +211,11 @@ $OSTREE checkout --atomic-retarget test2
 cd test2
 assert_file_has_content ./yet/another/tree/green "leaf"
 echo "ok checkout short form"
+
+cd ${test_tmpdir}
+rm -rf test2
+mkdir linkcache
+$OSTREE checkout --link-cache=linkcache test2 test2-checkout-from-link-cache
+cd test2-checkout-from-link-cache
+assert_file_has_content ./yet/another/tree/green "leaf"
+echo "ok checkout link cache"

tests/t0001-archive.sh

@@ -21,7 +21,7 @@ set -e
 
 . libtest.sh
 
-echo '1..18'
+echo '1..19'
 
 setup_test_repository "archive"
 echo "ok setup"
@@ -96,3 +96,11 @@ echo "ok fsck"
 cd ${test_tmpdir}
 $OSTREE checkout test2 checkout-test2-from-unpacked
 echo "ok checkout union 2"
+
+cd ${test_tmpdir}
+rm -rf test2
+mkdir linkcache
+$OSTREE checkout --user-mode --link-cache=linkcache test2 test2
+cd test2
+assert_file_has_content baz/cow moo
+echo "ok checkout link cache"