set fileEncoding in streamLogFileUnsafe

set fileEncoding in streamLogFileUnsafe

Windows: Fix bug that can cause git status to show annexed files as
modified when built with OsPath.

This may also have caused bugs on non-Windows, with filenames with
non-ascii characters? Unsure.

The OsPath conversion makes this one of the last few places
(hopefully) where a String is read from a Handle. All other fileEncoding
uses have been eliminated before this point by converting to reading
ByteString and using OsPath. Doing that here would be a better fix,
performance wise.

Sponsored-by: Jack Hill