Enable resolution of variables in config paths

[oWretch]

I have a desire to sync my configuration between machines where my username is different on each. Currently, the config backup just uses the raw path provided in the configuration file and doesn't attempt to resolve any variables from within the path.

This PR updates the config backup process to resolve variables in the config path string, such as ~ and $HOME and thereby allow one config to be used across different machines.

The upshot of this change is it would be possible for someone to perform advanced backup and restore by setting environment variables to move files around between machines.

[alichtman]

Hey! Thanks for the PR :) This fix makes sense to me.

I think it makes sense to break os.path.expandvars(os.path.expanduser(config_path)) out into a utility function. There are likely more places in the codebase we'd like to use that function. Once that change is made, this seems good to go.

EDIT

Turns out this code is already a util function:

shallow-backup/shallow_backup/utils.py

Lines 214 to 223 in 7d7cc3a

	def expand_to_abs_path(path):
	"""
	Expands relative and user's home paths to the respective absolute path. Environment
	variables found on the input path will also be expanded.
	:param path: Path to be expanded.
	:return: (str) The absolute path.
	"""
	expanded_path = os.path.expanduser(path)
	expanded_path = os.path.expandvars(expanded_path)
	return os.path.abspath(expanded_path)

Let's use expand_to_abs_path() here.

[alichtman]

I think this is also a problem for dest / quoted_dest.