D5477: branches: Added -r option to show branch name(s) of a given rev (Issue5948)

[Yuya Nishihara]

>   > ctx points to the tipmost branch head. so `rev not in revs` doesn't mean any
>   >  of the `revs` do not belong to the branch.
>   
>   Is there any possibility of workaround within this iteration itself @yuja? I can do outside the current loop by creating a branches list and map it by iterating over scmutil.revrange() as suggested by you. But, that won't fetch all details. Just changeset and corresponding branch. We need to keep the same output format, right?

Well, I meant you can build a set of branches to select.

```
selectedbranches = {repo[r].branch() for r in revs}
for tag, ... in repo.branchmap().iterbranches():
    if tag not in selectedbranches:
        continue
    ...
```

This is the simplest implementation. Since `repo[r].branch()` isn't a cheap
operation, it's probably better to query a revbranchcache.

```
getbi = repo.revbranchcache().branchinfo
selectedbranches = {getbi(r)[0] for r in revs}
```

There will be more optimization ideas (e.g. look up branchmap directly if
`len(revs) == 1`, but they won't be required to implement a minimal working
patch.