Add support for long (>260 char) file paths in Windows

msysgit has added support for long (>260 char) file paths in Windows, provided all clients and servers explicitly enable the core.longpaths option. They document: "Only enable this if you know what you're doing and are prepared to live with a few quirks." See https://github.com/msysgit/git/commit/c5f98452e98dca292c615000df7973ead63cf29b.

Stash doesn't support long file paths in Git repositories on Windows and has known issues if they are used, see https://confluence.atlassian.com/display/STASHKB/Stash+always+shows+incorrect+Merge+Conflict+in+PRs for example.

This suggestion is to improve Stash so that long file paths can be fully supported on Windows, provided the core.longpaths git option is enabled on the server and all clients.