Swap around test to defeat gcc 4.2's warning about i being unitialized

[dim]

Summary

Gcc 4.2 warns about the new diff:

usr.bin/diff/diffreg.c: In function 'change':
usr.bin/diff/diffreg.c:1085: warning: 'i' may be used uninitialized in this function

This warning is a little bogus, but indeed the way i is re-used in
this part of the function is a bit strange.

We can swap around the tests for diff_format being equal to D_EDIT
and i being zero, which fixes the warning.

Alternatively, we could just put i = 0 just above the previous if
statement. Let me know what you prefer.

[dim]

See D10484 for an alternative approach, which should be less confusing.