Make it easier to pass dirdiff args for one invocation

[danvk]

For a recent diff, I wanted git webdiff to be more aggressive about finding rename + change. git diff supports this via -M2 which only requires 20% overlap for a delete/add pair to count as a move.

The only way I could find to pass this to webdiff was to add this to my .gitconfig:

[webdiff]
  extraDirDiffArgs = "-M2"

It would be nice if there were an environment variable or command-line flag I could pass to set this for a single invocation.