Description of issue or feature request:

Judging from the docstrings and comments, add_target and add_targets are expected to update a Targets object with passed target paths, if they are relative to a targets directory base path (i.e. self._targets_directory).

However, they don't seem to take into account that Python's os.path.join ignores all path component arguments prior to a component that looks like an absolute path.

Current behavior:

• Works as expected (?) if passed target path is relative to a targets directory base path or absolute but falls into the targets directory:

# repo._targets_directory == "/path/to/repo/targets"
repo.targets.add_target("foo.txt")
repo.targets.add_target("/path/to/repo/targets/bar.txt)
# Successfully adds "foo.txt" and "bar.txt" 

• Fails if the passed target path is not absolute and does not exist relative to targets directory base path "/path/to/repo/targets".

# repo._targets_directory == "/path/to/repo/targets"
repo.targets.add_target("repo/targets/foo.txt")
# Fails with "'/path/to/repo/targets/repo/targets/foo.txt' is not a valid file in the repository's targets directory"

• Fails if the passed target path is absolute and does not exist

#repo._targets_directory == "/path/to/repo/targets"
repo.targets.add_target("/path/does/not/exist/foo.txt")
# Fails with "'/path/does/not/exist/foo.txt' is not a valid file in the repository's targets directory"

• Has unexpected behavior if the passed path is absolute and exists!

Expected behavior:
Revise intended behavior, update implementation accordingly.