Setting backup path to ~/.shallow_backup breaks subsequent runs

[ThatsJustCheesy]

I wanted my backup to be hidden, so I innocuously specified ~/.shallow-backup as my backup dir. The first backup ran fine, but subsequent runs spit out this message that explains the "what" but not the "why":

ERROR: /Users/ian/.shallow-backup is a directory, when we were expecting a file. Manual intervention is required.

This is apparently triggered by config file migration. If this code needs to remain for the forseeable future, then ~/.shallow-backup should probably be forbidden as a backup path.

[alichtman]

Yep, this seems like an unintended consequence of the config file migration.

I think that removing this set of lines will solve the problem without causing any other side effects:

shallow-backup/shallow_backup/upgrade.py

Lines 44 to 46 in fab050d

	elif os.path.isdir(old_config_path):
	print_red_bold(f"ERROR: {old_config_path} is a directory, when we were expecting a file. Manual intervention is required.")
	sys.exit(1)

Would you like to put up a PR?