v6.0.0

Changes

• The --exec/-x option does not spawn an intermediate shell anymore. This improves the
  performance of parallel command execution and fixes a whole class of (present and potentially
  future) problems with shell escaping. The drawback is that shell commands cannot directly be
  called with --exec. See #155 for the full discussion. These changes have been implemented by
  @reima (Thanks!).

Bugfixes

• --exec does not escape cmd.exe metacharacters on Windows (see #155, as above).

Other

• fd is now available in the FreeBSD ports (@andoriyu)
• The minimal rustc version is now checked when building with cargo, see #164 (@matematikaadit)
• The output directory for the shell completion files is created if it does not exist (@andoriyu)