do not convert local paths to URL

[smola]

• Do not convert local paths to URLs, just keep them as they
  are.
• This way we add support for Windows without taking care of
  Windows path-to-URL conversion.

[smola]

It seems it works :)

We used to get a lot of these errors in Windows tests:

----------------------------------------------------------------------
FAIL: references_test.go:291: ReferencesSuite.TestObjectNotFoundError
references_test.go:300:
    c.Assert(err, IsNil)
... value *plumbing.PermanentError = &plumbing.PermanentError{Err:(*url.Error)(0xc043723950)} ("permanent client error: parse file://C:\\Users\\appveyor\\AppData\\Local\\Temp\\1\\tmp-tgz-678061493: invalid character \"\\\\\" in host name")

not anymore

[alcortesm]

I love this.