Per-project branch switching

[aljazerzen]

Branch commands currently do not heed project_stash/database file. edgedb-rust does read it, so when connecting, it will be used, but branch switch does not write to it. But it should.

In other words, current branch should also be a per-project setting, not just per-instance.
When outside the project, instance's branch is used and when in a project, project's branch is used.

# cd project1
# gel branch current
main       (the project's branch, read from project stash)

# cd ..       (outside of any project)
# gel branch current
another    (the instance's branch, read from credntials.json)

# cd project2
# gel branch current
third-one

And we need to actually test this.