Make it so pin list display the current revision of the pinned repo

[desumn]

Implementation of issue #5533

It works, but I'm not sure it's the best way to do it

[desumn]

In particular, may the url or revision be empty for a git/hs/darcs kind?

[rjbou]

Thank you for you PR!
You'll find some comments specific to the implementation the implementation.
To complete the PR, it needs also to:

• add a test (we can do it)
• update the list manpage text to inform about the new column, especially that it is the hash of the current state of the source, not the installed one.
• polishing (remove trailing whitespace, indent, 80 columns), but we will do it at the end.

A todo for the dev team: we'll need to add a more exhaustive test to pin, existing one tests only the several kind of pin (vcs/path/version), but not the not synchronised package.

[kit-ty-kate]

side remark on the code itself: many of the lines of code added by this PR contain a trailing whitespace. It would be nice to avoid them

[desumn]

Thanks for your review, I'll be working on implementing them

[kit-ty-kate]

lgtm. Thanks for this contribution!
We'll do the rest (fix the tests)

[rjbou]

Discussed on dev meeting: We end up converging to this syntax

[rjbou]

Thanks!!