(GH-1603) Ensure full path for log when relative path provided
(GH-1603) Ensure full path for log when relative path provided

Use helper method in IFileSystem implementation to ensure that the full
path to the requested log file is used, rather than making it relative
to where Chocolatey is currently located.

TheBigBear/choco,TheBigBear/choco,jberezanski/choco,jberezanski/choco,TheBigBear/choco,jberezanski/choco