Do not create a new issue on cargo-generate merge conflicts if one already exists

[BD103]

Closes #276.

This uses gh to query for open issues, counts them, then exits early if any exist.

[benfrankel]

Small nits but looks good.

[BD103]

@benfrankel out of curiosity, why do you prefer to make all $VARIABLES as "${VARIABLES}"? As far as I can tell, they do the same thing.

[benfrankel]

@BD103 They help disambiguate in the general case. For example:

echo "$FOO-bar"
echo "${FOO}-bar"
echo "${FOO-bar}"

Ofc if you're just doing "$FOO", there's no disambiguation needed, but in that case I still prefer "${FOO}" for the consistency and so that I never have to even think "do I need {} here?", I just always add them.

Also see the Google style guide: https://google.github.io/styleguide/shellguide.html#s5.6-variable-expansion