I need to use release date versioning at least for now until preparing
custom version comparator for brew's formula.

Use extracted create_hook helper in tests for:

- exec
- hooks
- rehash
- version-name
- version-origin
- which

Spaces in hook path is tested in test/hooks.bats

`default` was made legacy back in 2011 with
5be66da9f4 (the command was renamed from
`rbenv-default` to `rbenv-global`, and so the global file was renamed
from `$RBENV_ROOT/default` to `$RBENV_ROOT/global` (the latter taking
precedence)

`global` was then made legacy about a month later in Sep 2011 when the
preferred filename was changed to `$RBENV_ROOT/version`.

References #730

It doesn't try to chdir into RBENV_ROOT anymore because that might be
a location of an unrelated rbenv install that might have a different
version than the current one that is installed e.g. via a package
manager such as Homebrew.

Now just tries the repo where the source files (`libexec/*`) are
located, and if that isn't a valid rbenv repo, bail out early.

No need for helper function that's gonna be used just once.

It's only supposed to be set from `version-origin` hooks, but not
inherited from environment in case it was set.

Expose a `version-origin` hook.

It is invoked *before* the traditional `rbenv-version-file` lookup. Because `version-origin` is traditionally run immediately after `version-name`, then any plugin hooks that alter `version-name` would have done so. Thus, running `version-origin` prior to printing the origin gives those plugins a chance to alter the `version-origin` to match.

If any of the hooks set `$RBENV_VERSION_ORIGIN`, then it is used as the return value. Otherwise, the existing logic continues to return "environment variable" or "filename" as appropriate.

This change, in conjunction with the `version-name` hook, makes a clean seam by which plugins can inject their own ruby version setting logic. Using this seam, as opposed to altering `$RBENV_COMMAND_PATH` from the `which` hook, means that the version name and origin are set more reliably and so `version`, `version-name`, `version-origin` and `which` all work as expected. Indeed, even PS1 works now.

Expose a `version-name` hook.

It is invoked *after* the traditional `RBENV_VERSION` lookup. Which means hook scripts can interrogate `$RBENV_VERSION_FILE` and/or `$RBENV_VERSION` (or use the executables).

The hooks are then run, giving plugins a chance to alter `RBENV_VERSION`. Once the hooks have run, we now have (in `$RBENV_VERSION`) the actual version we want to use (or it's empty which defaults to `system` per normal). Lastly, the same logic remains for checking if the version exists, or trimming the `ruby-` prefix.

Prime example: the ruby-bundler-ruby-version plugin can select a ruby by using the `ruby` directive from the `Gemfile` if a local `.ruby-version` doesn't exist.

Fixes #807

Can be used for `.ruby-version` file lookup in the ancestry of a
specific directory. In this mode of operation, global version files
aren't taken into consideration, and the command fails unless a local
version file was found.

Confirms #836

This new test was creating an (intentionally invalid) .ruby-version file
in current working directory; typically the rbenv project dir.
Immediately after test runs, I had a leftover .ruby-version file.

The version-file tests create and cd into the RBENV_TEST_DIR as part of
setup(). I'm using the same directory for this test fix, but am only
using it for this particular test. None of the other exec tests seem to
need to be in a temp test dir, so no use putting it in setup().

ensure subcommand exists, then include --help in its completion output

- Explicitly asking for help with `-h` or `--help` exits with 0 status
  and displays help on stdout.

- Not providing any arguments to rbenv results in failure status and
  displays version and help on stderr.

Broken in dcca61c0bc

If `foo` didn't exist and `RBENV_VERSION=system rbenv which foo` was
called, the error message used to be misleading:

    rbenv: version `system' is not installed

Instead, have the error message simply say that the command was not found.

Fixes #770

I was seeing /bin being left.  This patch fixes it by anchoring the search
pattern with ':' on both sides.

The symlinks weren't correctly resolved if they were pointing to a
single path component, such as `ln -s foo bar`.

Useful in combination with `--bare` to list just the unique version
numbers without the extra directory entries that are symlinks to other
version numbers in the same directory.

When `rbenv --version` is called, this now happens:

1. It changes into the directory where `libexec/rbenv--version` resides
   and checks if it's a checkout of the rbenv repo (as opposed to
   Homebrew checkout or something else). Then it reads the git revision.

2. If that failed, change to `$RBENV_ROOT` directory and repeat step 1.

If set by the user's environment, `git config --global` writes will go
to that destination instead of temporary $HOME. We definitely don't want
that.

Fixes #742