Interpret authors file path relative to the current directory at the time the command was executed

[jnm2]

This fixes the warning always seen when using --authors with a path that is not fully qualified:

Failed to copy authors file from "..\authors.txt" to ".git\git-tfs_authors".

I tried to set up a test but the existing fixtures don't seem to exercise a code path that copies the authors file. Let me know what to do if a test is needed.

[pmiossec]

Thanks