Why doesn't `git diff` invoke external diff tool?

Question

In my repository, if I type

$ git diff some-file

or

$ git difftool some-file

I get the in-terminal diff displ

Answer 1

It seems that git diff (at least, as of git version 1.7.12.4) will not run anything other than the internal, console-only diff on a file which is in the "both modified" state. git mergetool works on such files, though.