Make RET goto source file

[pgweiss]

Currently when typing RET in a diff buffer, I get
"Can't find filespec(s) in diff file header."
The reason is that the point is not positioned at the header.

[pgweiss]

Might resolve #180, if I'm interpreting it correctly.

[gareth-rees]

I can't merge this change because it breaks the existing behaviour, which is that typing RET in a diff goes to the corresponding line in the corresponding source file. (For example, if you have point on a line deleted from a file in some version, then typing RET goes to the deleted line in the previous version.) I don't want to break this behaviour, which is useful even if you occasionally get an error because point is in the wrong place and diff-mode can't deduce where it should be.

I recently upgraded to 24.5.1 and find that I can no longer reproduce #180. What version of Emacs are you using? Can you try upgrading to 24.5.1 and see if it fixes the problem for you?