Assert existence of config file's parent directory

Fixes TheLocehiliosan/yadm#226

When a config file path is passed to yadm whose parent directory does
not exist, git (and hence yadm) fails without writing the file.
Yadm should, however, make sure that the directory exists s.t. git can
just write to the file.

yadm

@@ -849,6 +849,8 @@ EOF
     CHANGES_POSSIBLE=1
 
   else
+    # make sure parent folder of config file exists
+    assert_parent "$YADM_CONFIG"
     # operate on the yadm configuration file
     "$GIT_PROGRAM" config --file="$(mixed_path "$YADM_CONFIG")" "$@"