fix windows path handling in blame view

[enizor]

revparse_single uses a git object name and not a file path - that means backslashes must be translated to slashes on windows.
Otherwise the blame view fails:

┌Blame -- src\main.rs -- <no blame available>

I reused the cfg! conditional from git2-rs/src/remote.rs, but there might be a better way to do that.

Resulting blame view on windows:

 Blame -- src\main.rs -- 869e4b7───────────────────────────────────────────────────────────────┐
 │f8f05d6 2020-03-31 Stephan Dilly          0│ #![forbid(unsafe_code)]                         │

[enizor]

The bug probably got introduced by #762 😅 I should have checked more thoroughly

[extrawurst]

we should probably refactor blame_file to take a Path (which automatically handles the separators) as a more sustainable solution

[extrawurst]

@enizor let me know if you need more help to bring this to the finish line. for the time being I mark this PR as draft, let me know when you think its ready

[enizor]

fixed by #984