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.

test/test_config.py

@@ -137,3 +137,27 @@ def test_config_local_write(runner, yadm_y, paths, supported_local_configs):
         assert run.success
         assert run.err == ''
         assert run.out.strip() == f'value_of_{config}'
+
+
+def test_config_without_parent_directory(runner, yadm_y, paths):
+    """Write and read attribute to/from config file with non-existent parent directory
+
+    Update configuration file
+    Display value
+    Exit with 0
+    """
+
+    config_file = paths.root + '/folder/does/not/exist/config'
+
+    run = runner(yadm_y('--yadm-config', config_file, 'config', TEST_KEY, TEST_VALUE))
+
+    assert run.success
+    assert run.err == ''
+    assert run.out == ''
+
+    paths.config.write(TEST_FILE)
+    run = runner(yadm_y('--yadm-config', config_file, 'config', TEST_KEY))
+
+    assert run.success
+    assert run.err == ''
+    assert run.out.strip() == TEST_VALUE

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")" "$@"