#### Optional arguments:
#### 'keep first' -- first file is removed unless this is present.
#### 'ignore space' -- spacing differences will not be considered
-#### signficant, emulating GNU diff -w. It is assumed that the
+#### significant, emulating GNU diff -w. It is assumed that the
#### first file has already been run through prepare_space.
#### ':' <test name> -- will print '<test name>' in the failure message,
#### to make it easier to tell which of multiple tests has failed.