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.

fail helper prints message to stderr and exits 1
(prefixes 'rbenv: ' if given a string arg)

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

If subcommand is provided (and exists) and its first arg is -h/--help,
go ahead and intercept the call; redirecting to rbenv-help <subcommand>

This means subcommands and plugins need not handle --help flag
themselves

Without it, it falls back to `--version` being executed as a subcommand,
which works but isn't obvious

- 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.

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

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.

BASH_SOURCE might be more reliable.

This is for Linux desktop platforms that have Terminal application
configured to start shells in interactive but not login mode. Creating a
`~/.bash_profile` would also cause `~/.profile` to not run, which might
be a problem on Ubuntu which ships with a default `~/.profile`.

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.

Closes #725

This is useful as an indicator where it is coming from.

`PYENV_FILE_ARG` is used here to make use of `abs_dirname` later in
`libexec/pyenv`.

Fixes https://github.com/yyuu/pyenv/issues/404

This is required for the shims to handle `#!/usr/bin/env python3` in a
shebang, just like `python` is handled currently: it will set
`PYENV_DIR` to the root of the invoked script, which is required for a
`.python-version` script to get picked up from there.

This was rejected for rbenv, where it does not make much sense
(https://github.com/sstephenson/rbenv/pull/735).

Ref: https://github.com/yyuu/pyenv/pull/368#issuecomment-102806837

It's not that this is a preferred way to set a global version (one
should use `rbenv global <version>` instead), but this fixes the
function purely for correctness: all parent directories should be
scanned, even the root directory.

Fixes #745

I was seeing the following occasionally in scripts:

> …/.pyenv/libexec/pyenv-version-file-read: line 12: type: write error: Broken pipe

This patch hopefully improves/fixes this, and it seems better anyway to
just use sed here.

This can be done using bash directly.