Batdiff can't read from stdin

`batdiff` should be able to read a diff from input and parse it through `delta` like `delta` itself does. This would allow setting `batdiff` as the pager for `git diff`.

Start at the batdiff entry point and trace how it currently receives input, then compare that flow with how delta reads and parses a diff. Verify the result with input from git diff, including configuring batdiff as the pager; done means a diff from standard input is parsed and displayed correctly.

Written by the indexing model from the issue text.