Revert "Work around issue using buffer number rather than name"

This reverts commit d6540b2588044f4028f6323e421e2e90116c8af7, which
caused all sorts of breakages with buffer names with brackets in them.
This was greatly exacerbated by airline.vim setting an erroneous
b:git_dir in plugin buffers based on the current working directory.

Closes #464. Closes #463. Closes #461.