Skip dependency filename options for object hash.

The filename of a dependency file doesn't have any bearing on the result, so
using it to calculate object hashes gives incorrect results.

This skips the <file> part of:
  -Wp,-MD,<file>
  -Wp,-MMD,<file>
  -MF<file>
  -MF <file>

Also tests for this functionality.

Fixes issue jrosdahl#28.