Compute `changed` when `state: latest` by diffing `LINK` and `UNLINK` lines in verbose stderr

[saulrh]

Right now, changed is always true when re-stowing because stderr is always populated. It should be possible to tell when something actually changed by looking at the verbose output on stderr:

• Any line that matches ^LINK but not (reverts previous action)$ is a "real" change, because it's a new file.
• Any line that matches ^UNLINK but doesn't have a corresponding line matching ^LINK is a "real" change, because it's file that disappeared.
• Any line that doesn't match ^LINK or ^UNLINK indicates something else that might or might not be a change and should probably just be a change.

[saulrh]

Proof of concept would look something like this (untested):

unlink_re = re.compile('^'
                       'UNLINK: '
                       '(?P<link_path>.+)'
                       '$')
link_re = re.compile('^'
                     'LINK: '
                     '(?P<file_path>.+)'
                     ' => '
                     '(?P<link_path>.+)'
                     '(?P<reverts> \(reverts previous action\))?'
                     '$')

unlinks = set()
links = set()
for line in _se:
    result = re.match()
    if result:
        unlinks.add(result.group('link_path'))
for line in _se:
    result = link_re.match()
    if result:
        links.add(result.group('link_path'))
changed = unlinks == links

[upsetbit]

Could you look the PR I sent? Didn't have much time to work on this but it yield some OKish results. Needs to be more stressed.