CLI: jj root vs jj (workspace|git) root

[averms]

I'd like to call attention to what I as a new user of jj think is a minor inconsistency
in the CLI. In commit a39dd0f, jj workspace root was
duplicated to jj root. The commit message seems to suggest that the task of printing
different types of root paths should be under the purview of jj root:

cli: duplicate cmd_workspace_root() to cmd_root()

Suppose cmd_root() will gain options to print other kind of root paths, it's
probably simpler to inline path handling there instead of turning cmd_root()
into a dispatcher function. If cmd_root() remains with the current form, maybe
we can add aliases.root = ["workspace", "root"] instead.

However, that is not currently the case. jj root prints the workspace root only and
the options for printing other kinds of roots are spread out into the subcommands of jj workspace and jj git.

In short, the current state is that jj root exists but is limited, while other root
types are found under separate command groups (jj workspace, jj git). This leads to
a discoverability problem for new users:

1. Seeing a command named simply jj root intuitively suggests it might be a general
   command for finding any kind of relevant root path in the repository or workspace.
2. Typing jj root --help confirms it only does the workspace root and provides no hint
   about where to find the command for the git root.
3. A user specifically looking for the git root might incorrectly conclude that jj
   doesn't offer this functionality via the CLI, purely because it's not under the
   logical root command or cross-referenced in its help text. They would need to know
   to look under the jj git subcommand.

I think that either jj root should be removed, or it should grow the ability to print
git roots too.

[yuja]

Some ideas were discussed in #2801, but I don't have strong feeling. I personally don't like jj root --git thingy because it's not easy to extend the jj root command to support other backend implementations.