Add "preexec" from "https://github.com/rcaloras/bash-preexec@0.4.1"

git-vendor-name: preexec
git-vendor-dir: vendor/github.com/rcaloras/bash-preexec
git-vendor-repository: https://github.com/rcaloras/bash-preexec
git-vendor-ref: 0.4.1

vendor/github.com/rcaloras/bash-preexec/test/README.md

@@ -0,0 +1,22 @@
+Testing `bash-preexec`
+======================
+
+**Note on test conditions**
+
+When writing test conditions, use `[ ... ]` instead of `[[ ... ]]` since the
+former are supported by Bats on Bash versions before 4.1. In particular, macOS
+uses Bash 3.2, and `[[ ... ]]` tests always pass on macOS.
+
+In some cases, you may want to use a feature unique to `[[ ... ]]` such as
+pattern matching (`[[ $name = a* ]]`) or regular expressions (`[[ $(date) =~
+^Fri\ ...\ 13 ]]`). In those cases, use the following pattern to replace “bare”
+`[[ ... ]]`.
+
+```
+[[ ... ]] || return 1
+```
+
+References:
+* [Differences between `[` and `[[`](http://mywiki.wooledge.org/BashFAQ/031)
+* [Problems with `[[` in Bats](https://github.com/sstephenson/bats/issues/49)
+* [Using `|| return 1` instead of `|| false`](https://github.com/bats-core/bats-core/commit/e5695a673faad4d4d33446ed5c99d70dbfa6d8be)