Do not overwrite HISTFILE about mcfly

[cantino]

Thanks @a-dekker, that's a good addition.

[cantino]

I'll include a fix for this in #81.

[a-dekker]

It doesn't seem to be fixed, it still contains the line
export HISTFILE="${HISTFILE:-$HOME/.bash_history}"
at https://github.com/cantino/mcfly/blob/master/mcfly.bash#L13

[cantino]

Hey @a-dekker, sorry about that. I actually reverted the change, as it was giving me odd issues in BASH where the variable didn't seem to be set.

[a-dekker]

That seems strange, never experienced any issues with the alternative implementation.
The change is also very straightforward: test if the value is set, if not, set the default.
It should do the same as your implementation, except not overwrite it by default.