Bug/FR: `jj git fetch --branch main` should fetch tags

[ilyagr]

Currently, jj git fetch --branch main does not fetch any tags from the remote. (jj git fetch without --branch fetches all the tags.)

jj git fetch --branch could match git fetch's default behavior of fetching all the tags that point to ancestors of commits being fetched.

It's unclear to me how difficult this is. For example, would we have to filter tags ourselves or does libgit2 implement this functionality?

[yuja]

libgit2 has the option, but it's "auto download" by default. So it might be difficult to fix if the option isn't working.
https://docs.rs/git2/latest/git2/struct.FetchOptions.html#method.download_tags

btw, we have another bug at our side that prevents tags from imported by jj git fetch. we'll need to run another jj git import to make tags visible.