This is so the tags from the automatically-added remote (typically
someone's fork) don't spill in and mix with the current tags.

It is `headRepo.Owner.Login` that holds the unix account name of the
owner, and `headRepo.Owner.Name` is omitted from the embedded owner
information, resulting in the following failure:

```
% hub checkout https://github.com/github/hub/issues/{number}
fatal: '' is not a valid remote name
```

To obtain the owner/fork-name combo, instead of reading it from "label"
field from the PR response simply read from `Head.Ref` and
`Head.Repo.Owner.Name` instead. This is because label is meant to be
human-readable, isn't well-documented in GitHub API docs and is unclear
when it contains two components (delimited by `:`) and when it contains
only one.

`Args.InsertParam` panics about index out of bound if the index is larger than param size. These changes reset the index to param size for such case. This is equivalent to appending new items to the end.

This fixes #812.

Start reading the `Head.Repo.Name` property from API payload to
determine the name of the forked repo which might not match the name of
the base repo.

- Use a hash to lookup for commands.
- Add more tests for the command and runner execution.

Since https://github.com/github/hub/compare/31a85e519e0a080dd10ce21339f806a9a0877766...master

Some tests are ignored either because they don't make sense or WIP