Fix diagnostic formatting regression caused by adaa91a33eb2cf23b88.

Ninja regressed to include a location for every file on the include stack for
nested diagnostics, i.e. it would print:

  input:1: include.ninja:1: expected path

Fix this so that it prints only the current file location, like it used to:

  include.ninja:1: expected path

Also add a test for this.
Merge pull request #623 from nico/fix

Fix diagnostic formatting regression caused by adaa91a33eb2cf23b88.