Increase swiper-isearch cursor overlay priority

[jrosdahl]

Otherwise the swiper match overlays can make the cursor disappear when using swiper-isearch-backward.

Priority 2 seems to work for me, but I haven't really tried to understand why or if it should be higher. Feel free to bump it.

How to reproduce

Using the default ivy--regex-plus re builder:

1. Visit a buffer with the text "foo bar".
2. swiper-isearch-backward
3. Enter search expression "foo bar".

Actual behavior

4. The cursor overlay in the buffer disappears when "b" is entered.

Expected behavior

4. The cursor overlay in the buffer is always visible at the start of the match.

[abo-abo]

Thanks.