[MSBuild]::MakeRelative works completely incorrect with Unix paths that contains "up-directory" symbol in.

In case
`[MSBuild]::MakeRelative("\foo\bar\..\abc\cde\", "\foo\bar\..\abc\fff\ggg.hh")`

the function returns
`\foo\bar\abc\fff\ggg.hh`

When accorring to https://msdn.microsoft.com/en-us/library/dd633440.aspx the correct result has to be
`..\fff\ggg.hh`

Note I am using '\\' as directory separator because #429 though it has to be '/' for Unix systems.