cmd: --branch option for git fetch.

[ilyagr]

The main contribution here are the detailed tests.

There is an outstanding bug with the way undo interacts with jj git fetch --branch.

Checklist

Todo:

• I have updated CHANGELOG.md
• (not planned) I have updated the documentation (README.md, docs/, demos/)
• I have added tests to cover my changes

[ilyagr]

FYI, I have a synced-to-head version of this at https://github.com/ilyagr/jj/tree/fetch-glob.

The test there is a union of the test from head and my test, and needs to be merged more intelligently.

It's buggy enough that I couldn't put it here easily (I haven't tried doing jj git fetch --remote upstream without --glob yet).

$ jj git push --remote upstream -b ig/fetchglob
Branch changes to push to upstream:
  Add branch ig/fetchglob to c20c34f0e914
Error: Push is not fast-forwardable

[ilyagr]

There are a couple of known outstanding bugs (search for BUG in the tests). However, I think it's better to check this in as is rather than leave it hanging. I'll file these bugs as I submit this.

[samueltardieu]

I am all for including this soon, this is a feature I will use very often when pulling a pull request from a third-party repository without getting the whole content.

There is one thing I'm unsure about (the refspec) and I've let a comment on it, I would be more confident if you included the output of get_branch_output() at several points in the test.

[ilyagr]

Thanks Samuel!