Use sign groups

[pmhahn]

"Coveragepy noshow" and "... refresh" did have no effect for me.

Vim supports named group: https://vimhelp.org/sign.txt.html
Use them to group "[branch]uncovered" signs into them as it simplifies
clearing previous signs.

Switch "s:ClearSigns()" to use buffer number instead of file name.

This also makes #25 obsolete as it also clears the branchuncovered signs.

[alfredodeza]

Thanks for the PR. I'd forgotten about this issue and had a local fix which I never pushed/released. Vim 8 changed the output of this:

let placements = split(execute('sign place file=' . bufname('%')), '\n')

And so the split() that followed was broken. I verified the groups change and it makes way more sense. Thanks!