`grep-cli` may run arbitrary executables on Windows

On Windows in versions of grep-cli prior to 0.1.6, it’s possible for some
of the routines to execute arbitrary executables. In particular, a quirk of
the Windows process execution API is that it will automatically consider the
current directory before other directories when resolving relative binary
names. Therefore, if you use grep-cli to read decompressed files in an
untrusted directory with that directory as the CWD, a malicious actor to could
put, e.g., a gz.exe binary in that directory and grep-cli will use the
malicious actor’s version of gz.exe instead of the system’s.

This is also technically possible on Unix as well, but only if the PATH
variable contains .. Conventionally, they do not.

A DecompressionReader has been fixed to automatically resolve binary names
using PATH, instead of relying on the Windows API to do it.

If you use grep-cli’s CommandReader with a std::process::Command value
on Windows, then it is recommended to either construct the Command with an
absolute binary name, or use grep-cli’s new
resolve_binary
helper function.

To be clear, grep-cli 0.1.6 mitigates this issue in two ways:

• A DecompressionReader will resolve decompression programs to absolute
  paths automatically using the PATH environment variable, instead of relying
  on Windows APIs to do it (which would result in the undesirable behavior of
  checking the CWD for a program first).
• A new function, resolve_binary, was added to help users of this crate
  mitigate this behavior when they need to create their own
  std::process::Command. For example,
  ripgrep uses grep_cli::resolve_binary
  on the argument given to its --pre flag.

While the first mitigation fixes this issue for sensible values of PATH
when doing decompression search, the second mitigation is imperfect. The more
fundamental issue is that std::process::Command is itself vulnerable to this.