There exist cases within Git LFS where we would like to perform a
"pseudo-clean" on a given filepath. It should behave similarly to a call
to filepath.Clean, but should only remove the beginning "./" (macOS,
*nix) or ".\" (Windows, etc).
This patch introduces trimCurrentPrefix, which removes such prefixes.
It will be used in subsequent patches to perform prefix-independent
comparison against arguments to "git lfs track" and "git lfs untrack"
and the contents of a repository's .gitattributes file.
Since we assume that the contents of ".gitattributes" are written on
multiple platforms, each with their own platform-specific prefixes,
trimCurrentPrefix is instructed to remove _all_ prefixes, not just the
current platforms.
