brancher: potential collision with "workspace" tags/branches

[efiop]

#3914 (comment) Kudos @Suor

[Suor]

There are two subissues caused by this name clash:

Input one. When someone uses repo.brancher(revs=[..., "workspace"]) it's unclear whether he or she wants workspace to be included or there is a tag/branch named "workspace", which needs to be included.

Output one. When brancher yields "workspace" does it show workspace or a tag/branch with that name? This further confuses things like metrics/plots show/diff output and corresponding Repo.metrics.show()/Repo.params.show().

Also, we sometime refer to workspace as "" (empty string) for whatever reasons.

[efiop]

We could mark it with some char that is forbidden for tags. With working tree it was the space that was illegal, so we didn't have any issues. We could do something similar by including some rare illegal character somewhere or just using a flag when browsing and then on collision add something like workspace(working tree)(or tag/branch) or something.

[Suor]

If we don't care about presentation we can make some str subclass:

class Workspace(str):
    pass
WORKSPACE = Workspace("workspace")

Will work both for in and out. Looks as hacky as special char to me.

On the other hand the most clean solution I can think of is use flag for input and empty string for output. CLI should show workspace or something instead of empty string and it should go first.