Support diverging push/pull repo configuration better

### Is your feature request related to a problem? Please describe.
When I contribute to big open source projects, I have come to like a configuration like this:

```
❯ git remote -v |rg origin
origin git@github.com:NixOS/nixpkgs.git (fetch)
origin git@github.com:dwt/nixpkgs.git (push)
```

that is, I like to configure the push url to point to my clone, and leave the pull url to point to the upstream repo. On the command line, this allows me to interact with the repo with just `git fetch/pull` and `git push` without having me to distinguish that this is s an open source project I am contirbuting to or my own project.

In lazygit, this leads to all my feature branches to show up as missing (since they are not available on the pull side of the remote), even if they are perfectly pushed to my remote.

This also makes it harder to quickly check which of them have already been merged. (If I use the workflow that for merged pull requests, I click the GitHub button to delete my pull request).

### Describe the solution you'd like
I would like the 'remote available' display to be based on if the branch is available on the push remote, not on the pull remote.

### Describe alternatives you've considered
... not relying on this feature has been my goto for now, but it's kinda annoying.