In diff-mode, "o" doesn't recognize unified diff file headers if they do
not contain tabs. Some diff programs will not emit tab + timestamp (as
GNU diff does), but just the file name.

Example working file header (where ".txt" is followed by tabs):

--- a.txt 2015-04-08 00:09:46.000000000 +0200
+++ b.txt 2015-04-08 00:09:51.000000000 +0200

Example failing file header:

--- a.txt
+++ b.txt

The fallback is for "o" to open the file on the leading "diff" line
which may or may not be present.

This used to work in Emacs 24.3.1 but does not as of the patch mentioned
here:
http://stackoverflow.com/questions/14720205/how-do-i-use-emacs-diff-with-spaces

My proposal is to fix this in (diff-hunk-file-names) by removing the
requirement for the trailing tab on the leading diff file header and
replace it with a no-newline check (sorry for not providing proper
patch):

-          (if (looking-at "[-*][-*][-*] \\([^\t]+\\)\t.*\n[-+][-+][-+]
\\([^\t]+\\)")
+          (if (looking-at "[-*][-*][-*] \\([^\t\n]+\\).*\n[-+][-+][-+]
\\([^\t\n]+\\)")

The return values should probably use (match-string-no-properties)
rather than (match-string) as well.

Thanks,
 Gustav