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Invoke `env` via `command` to ignore shell functions and aliases.

This allows you to use `colored git log --help` for example, to get
colored output.

Resolves: #6237

Commit 09d2a597 introduced the bug, which is due to using
single brackets conditions at the same time as *.

This commit should fix it and use double brackets everywhere
else.

Fixes #4699

Some environments (in my case, cygwin) have PATH variables with directories
with spaces in them, breaking colored-man.
Adding quotes around the PATH environment variable makes the plugin run OK.