config-bot: support full path in skip-files

[jlebon]

With coreos/fedora-coreos-config#740, we'll want to sync everything except ci/buildroot/Dockerfile, but skip-files doesn't support full paths right now, just top-level entries. Briefly played with

diff --git a/config-bot/main b/config-bot/main
index 802890a..ca69b17 100755
--- a/config-bot/main
+++ b/config-bot/main
@@ -257,8 +257,8 @@ async def propagate_files(cfg):
                 continue

             # get the list of files from the source ref
-            all_files = git.cmd_output('ls-tree', source_ref,
-                                       '--name-only').splitlines()
+            all_files = git.cmd_output('ls-tree', source_ref, '--name-only',
+                                       '--full-tree', '-r').splitlines()
             files_to_import = [f for f in all_files
                                if not matches_patterns(f, skip_files)]

But all those files are then passed as cmdline args to git checkout which is hacky. We should find a cleaner way to have rsync --delete-like semantics.