FR: preserve current Git branch in co-located repo when possible

[ilyagr]

This report stems from the discussion starting with #2183 (comment).

In a co-located repo, jj generally puts the git repo in a "detached HEAD" state since jj does not have a concept of the current branch. In general, I don't propose we change that.

However, after a git switch branch, we could make more effort to preserve branch as a current branch when possible.

jj already makes some effort here. For example touch qq && jj log keeps Git's current branch at branch. This is good.

However, jj describe -m qq currently switches to a detached HEAD state, even though it doesn't modify the @- commit where branch is at. This is undesired.

jj new @- also currently switches to a detached HEAD state. Arguably, it shouldn't for the same reason: if the user just did git switch branch, they probably mainly care about whether the @- commit (where branch is at) changed.

For reference, the situation git switch branch brings us to (after the subsequent automatic jj git import) is:

@ (empty) (no description set)
|
o branch
|
...

[necauqua]

As a bonus it will get rid of annoying warnings in tools 🙃

(at least sometimes)

[isinyaaa]

Hi, there, could this be assigned to me and @wmrmrx?

[wmrmrx]

Commenting so I can be assigned @wmrmrx