lib/core: Support <remote>: syntax when listing refs

Allow users to pass `<remote>:` to list all refs we have locally
belonging to `<remote>`. Also (re-)allow the similar `<remote>:.` syntax
for backwards compatibility with flatpak.

Closes: #1500
Approved by: cgwalters

tests/test-refs.sh

@@ -147,6 +147,13 @@ ${CMD_PREFIX} ostree --repo=repo refs local1 --create=origin:local1
 ${CMD_PREFIX} ostree --repo=repo refs | wc -l > refscount.create6
 assert_file_has_content refscount.create6 "^11$"
 
+#Check that we can list just remote refs
+${CMD_PREFIX} ostree --repo=repo refs origin: | wc -l > refscount.create7
+assert_file_has_content refscount.create7 "^2$" # origin:remote1 origin:local1
+#Also support :. for backcompat with flatpak
+${CMD_PREFIX} ostree --repo=repo refs origin:. | wc -l > refscount.create8
+assert_file_has_content refscount.create8 "^2$" # origin:remote1 origin:local1
+
 echo "ok refs"
 
 # Test symlinking a ref