apply: reject a hunk that does not do anything

apply: reject a hunk that does not do anything

A hunk like this in a hand-edited patch without correctly adjusting
the line counts:

     @@ -660,2 +660,2 @@ inline struct sk_buff *ieee80211_authentic...
             auth = (struct ieee80211_authentication *)
                     skb_put(skb, sizeof(struct ieee80211_authentication));
     -       some old text
     +       some new text
     --
     2.1.0

     dev mailing list

at the end of the input does not have a good way for us to diagnose
it as a corrupt patch.  We just read two context lines and discard
the remainder as cruft, which we must do in order to ignore the
e-mail footer.  Notice that the patch does not change anything and
signal an error.

Note that this fix will not help if the hand-edited hunk header were
"@@ -660,3, +660,2" to include the removal.  We would just remove
the old text without adding the new one, and treat "+ some new text"
and everything after that line as trailing cruft.  So it is dubious
that this patch alone would help very much in practice, but it may
be better than nothing.

Signed-off-by: Junio C Hamano <gitster@pobox.com>