Update borgbackup to 1.1.2.

Full changelog (long) at
https://github.com/borgbackup/borg/blob/1.1.2/docs/changes.rst.

When upgrading from borg 1.0.x to 1.1.x, please note:

- borg might ask some security-related questions once after upgrading. You can
  answer them either manually or via environment variable. One known case is if
you use unencrypted repositories, then it will ask about a unknown unencrypted
repository one time.
- your first backup with 1.1.x might be significantly slower (it might
  completely read, chunk, hash a lot files) - this is due to the --files-cache
mode change (and happens every time you change mode). You can avoid the
one-time slowdown by using the pre-1.1.0rc4-compatible mode (but that is less
safe for detecting changed files than the default). See the --files-cache docs
for details.


(bsiegert)