Avoid using same command for querying and mutating (e.g. branches)

[martinvonz]

Description

@arxanas shared this confusing snippet:

$ jj branch help
Nothing changed.
$ jj branch delete foo 
warning: Updating multiple branches (2).
$ jj log
@ 617f177b78c7 ef64c980ded3 (no email configured) 2022-05-18 17:32:58.000 -07:00 delete foo forget help  
| (no description set)

I agree that that behavior is confusing. Git and Mercurial behave the same way. However, in their defense, they don't have subcommands at all, so the user is much less likely to think that using branch delete for deleting a branch would work. Since we do have subcommands, we may want to use them more consistently and never (?) have the same command either query or mutate the repo (except for the automatic working copy snapshots) depending on arguments.

@arxanas suggested having branch create, branch delete, and branch list (replacing the current branches). That sounds good to me, but I'm happy to hear what others think.

Specifications

• Version: 0.4.0