The previous implementation failed to prevent duplicates if the entry
being added was the only entry already present in the path. The reason
is that it always required a : to be present in order to remove the new
entry before adding it.
The new implementation is clearer and solves that. However, if the new
entry is already present it will be kept where it was, not moved to the
front of the list. This is fine at the moment.
SC2123 warns that the PATH variable is being manipulated, but this is
exactly the intention here.
SC1091 warns that /etc/profile cannot be checked. This is fine, as this
file is provided by the platform and there is no need to check it here.
shellcheck(1) is warning that the single quotes prevent the expression
from being expanded. However, this is the desired behavior because this
expression is meant to be interpreted by vim(1), not bash(1).
This makes sure the return code of extracting the path from manpath(1)
is not ignored. Also, unset the MANPATH environment variable instead of
just assigning an empty string to it, so that manpath(1) determines a
suitable default hierarchy to search for manual pages.
The way I was doing the test is incorrect, as [ is the test(1) program
and the shell builtin command is not something it understands. Instead,
I now just let the shell execute `command` and then set the variable if
`brew` is available.
