Separate the corner-cutting code for `rehash' into a dedicated branch.

Saves some more time (about 100ms for the large fixture)

provides a reliable way for Pyenv code to locate other parts of Pyenv code that are not under `PYENV_ROOT`

* tests: Work with Git entirely in a test repo

Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

Co-authored-by: Ivan Pozdeev <ivan_pozdeev@mail.ru>
Co-authored-by: Ivan Pozdeev <vano@mail.mipt.ru>

use global setup with bats with possibility to add specific test file _setup
use readlink in helper if realpath is not working as expected

Make sur that PYENV_TEST_DIR is created if mktemp failed
mock python3 usage by updating create_executable function
fix test by adding the right PATH
update path in plugin test
replace for loop with while read
don't use symlink for stub

* perf: replace a series of if statements with a case block. Add error
handling for case where unknown option is provided.
Same setup as rbenv-init for reading arguments.

* perf, docs: Recommend users to specify the shell for `pyenv init -`
Speeds up the startup by about 40% (in local testing, from ~50ms to ~30ms).
Reflect this in `pyenv init` hint text.

* style: remove unnecessary `root` variable in pyenv-init
* style: remove unnecessary variable declarations at the top of file in pyenv-init.

* perf: replace `cat <<` calls with `echo`
The builtin `echo` is about 100x faster. In tests, saves about 2-3ms.

* docs: document the `pyenv init - <shell>` performance boost in the Advanced Configuration section.

* style: test_helper.bash: avoid unnecessary ".." in produced PATH

* docs: fix a false statement about completions location in the Advanced Configuration section.

Required for diagnostics

`which` has been deprecated in Debian's debianutils 5.0+
They suggest using `type` and `command` as replacements:
3a8dd10b45 ,
610a65c6ef

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

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

With `realpath` extension, hooks tests on OS X will output
`/private/tmp` instead of `/tmp` because the latter is an actual symlink
to the former.

Avoid this mistmach in output assertions by expanding BATS_TMPDIR if
`realpath` extension is compiled.

This prevents the tests from removing an existing directory.

Fixes #561 (https://github.com/sstephenson/rbenv/issues/561)

On other systems, we expected to find system Ruby in `/usr/bin`, but in
OpenBSD 5.4 it will be found in `/usr/local/bin`.

This replaces the limited USRBIN_ALT hack with a more generic
`path_without` function that will ensure that the given executable is
not present in the resulting PATH even if it's found in multiple
system paths.

The error was "bash: no such file or directory" and it was due to bash
being located in `/usr/local/bin` on OpenBSD 5.4 instead of `/bin` like
on other systems.

Fixed by keeping `/usr/local/bin` in PATH during the test run.

It was a dumb idea and it wasn't even implemented perfectly.