For now the code is using traditional pseudo locking mechanism
based on `noclobber`.

Command `rbenv version-name > .ruby-version` will create an empty `.ruby-version` file
before running `rbenv-version-file`. This causes `rbenv-version-file` to return empty
string which in turn causes `rbenv-version-name` to return `system`.

Ensure size of `.ruby-version` is non-zero as a workaround.

Shell integration is not enabled by default. This means that, from all the
commands from `rbenv commands`, only "shell" won't work right away.

Replace "no such command" with a more descriptive message that points to
`rbenv init` instead.

Specifically, this does not play well with PWD=/proc/self/cwd

Forwarded from https://github.com/pyenv/pyenv/pull/606.

Revert back to original quoting style used before previous commit.
Adjust init.bats to reflect changes for successful tests.

The setenv function in fish shell has changed dramatically in
75600b6b53
It now conforms to the csh version, which takes at most two arguments.
In this init script, the form
    setenv PATH prepend_something $PATH
had been used, which had too many arguments.
Since setenv isn't a native command in fish, a suitable replacement is
to use the "set -gx" command, which can consume multiple arguments.

This prevents CDPATH from becoming an issue.

Closes #866.

- some versions of bash (e.g. 4.3.11) complain about clobbering
  /dev/null, use bash's explicit >| operator to ignore noclober
- not sure if the original #759 report has the same cause
- https://github.com/rbenv/rbenv/issues/759#issuecomment-289326891

* Update version to 1.0.9

Keeping rbenv-controlled variables to RBENV_* "namespace" helps with
discoverability (and tools like rbenv-env) but also consistency and a
very minor degree of safety/isolation from env impact.

This ensures that OLD_RBENV_VERSION is never exported. This makes the
implementation a little bit more complex, since more logic needs to be
pushed down into eval'd code.

The literal tilde in a PATH entry (e.g. `~/.rbenv/shims`) doesn't seem
to be supported by system `which` utility, but *does* seem to be
supported by `command -v` (used in `rbenv-which`) and `type -p`.

Therefore, we must strip away `~/.rbenv/shims` from PATH when looking up
executables for system Ruby, lest we risk infinite loop. We do so by
substituting any occurence of `~` in PATH with the value of `HOME`.