git: handle Windows junctions properly

On Windows, there is the concept of a junction point, which is a by-name
reference from the junction point to a directory using an absolute path.
Go's filepath.EvalSymlinks resolves this and provides a canonical path.
However, Git for Windows ignores them and provides a path containing
junctions, despite the fact that Unix versions of Git always provide an
absolute path.

When locking, we canonicalize paths on Windows before making them
relative to the root, meaning that if junctions are used, the lock path
contains a ".." component, which breaks unlocking. Canonicalize the
paths we get from Git so that we can be confident that lock paths will
be relative.

This case cannot, unfortunately, be easily tested in our testsuite,
since the Git Bash environment lacks support for invoking the system
mklink.exe utility, which is used to create junctions.

git/git.go

@@ -626,7 +626,11 @@ func GitAndRootDirs() (string, string, error) {

func canonicalizeDir(path string) (string, error) {
	if len(path) > 0 {
		return filepath.Abs(path)
		path, err := filepath.Abs(path)
		if err != nil {
			return "", err
		}
		return filepath.EvalSymlinks(path)
	}
	return "", nil
}