ui Printf functions: handle errors by dying

Closes #1979, fixes #1560

Replace deprecated Dial with DialContext

Improve `hub create` dealing with an existing "origin" remote

- No longer says "updating git remote", but not actually doing anything
- Shows a helpful warning about "origin" pointing to a possibly unrelated repo

Don't choke on literal `%` output from `--format` use

In general, avoid using Printf without a static format string.

Capitalize GitHub in doco

Clarify long-form CLI options

The previous format of documentation made it look like `--message VALUE`
(no equal sign) would be supported, which might be misleading.

add pullrequest vim syntax.

Add more information about merged PRs in `hub pr list`

This queries by `state=closed` and selects only PRs that have a merged date.

Restore pre-2.20.0 `git help` output

After upgrading to Git 2.20.0, all operations that relied on `hub alias`
would indefinitely hang. It can be reproduced using the following:

1. Create a new alias such as `st` for `git status`.
2. Run `hub <alias>` in a git repository.

  ```
  $ hub st

  ... hangs forever
  ```

Inspecting the running process shows that it is executing `git help -a`
and not able to proceed. Digging into the `hub` shows that the `help`
command is called within `IsBuiltInGitCommand`. Once this entrypoint was
found, I started bisecting and found this works in 2.19.x but fails in
2.20.x. Sure enough, the Git 2.20.0 release notes mention a
corresponding change[1].

>  * "git help -a" now gives verbose output (same as "git help -av").
>    Those who want the old output may say "git help --no-verbose -a".

It was even nice enough to include the backwards compatibility fix :)

To fix the hanging, I've restored the existing behaviour by including
the `--no-verbose` flag that mimics the 2.19.x behaviour.

Fixes #1963

[1]: https://github.com/git/git/blob/master/Documentation/RelNotes/2.20.0.txt#L17-L18

Add support for multiple `--message` options

This is for compatibility with git-commit(1)

It's not only the first line of text; it's the entire first "block" (or
paragraph) of text with newlines turned into spaces. This is for
compatibility with git-commit(1).