scmutil: remove special handling of pats==("",) in matchandpats() (API)
I don't know under what circumstances we used to pass that value, but
we don't seem to do it now.
Differential Revision: https://phab.mercurial-scm.org/D5921
$ cat >> $HGRCPATH << EOF
> [extensions]
> fastannotate=
> EOF
$ hg init repo
$ cd repo
changes to whitespaces
$ cat >> a << EOF
> 1
>
>
> 2
> EOF
$ hg commit -qAm '1'
$ cat > a << EOF
> 1
>
> 2
>
>
> 3
> EOF
$ hg commit -m 2
$ hg fastannotate -wB a
0: 1
0:
1: 2
0:
1:
1: 3