The simple filepath.IsAbs() is not enough on Windows, where the relative
path may start with a separator and should then be taken relative to the
volume name.
So, introduce resolveGitPath() helper function that will do the right
thing on both Windows and other systems. As a bonus, it returns paths
converted to slashes for ease of use with the rest of the git segment:
- Git for Windows uses only slashes.
- Slashes do work as a path separator on Windows anyway.
- Some tests in git segment still use (and rightfully so) slashes.
Add tests for resolveGitPath() on both types of systems and fix
TestEnableInWorktree using a system-dependent testing constant as a root
path.
