Navigation and use of diff buffers had several annoying corner cases that this
patch fixes. These corner cases were largely due to inconsistent treatment of
file headers. Say you have a diff such as this:

 --- aaa
 +++ bbb
 @@ -52,7 +52,7 @@
 hunk1
 @@ -74,7 +74,7 @@
 hunk2
 --- ccc
 +++ ddd
 @@ -608,6 +608,6 @@
 hunk3
 @@ -654,7 +654,7 @@
 hunk4

The file headers here are the '---' and '+++' lines. With the point on such a
line, hunk operations would sometimes refer to the next hunk and sometimes to
the previous hunk. Most of the time it would be the previous hunk, which is not
what the user would expect. This patch consistently treats such headers as the
next hunk. So with this patch, if the point is on the '--- ccc' line, the point
is seen as referring to hunk3.

Specific behaviors this fixes are:

1. It should be possible to place the point in the middle of a diff buffer, and
press M-k repeatedly to kill hunks in the order they appear in the buffer. With
the point on hunk1, M-k M-k would kill hunk1 then hunk2. With the point on
hunk3, it would kill hunk3 then hunk4; this is fine. However, with the point on
hunk2, it'd kill hunk2 then hunk1. This is fixed by this patch.

2. Similarly, it should be possible to apply hunks in order. Previously with the
point at the start, C-c C-a would apply the hunk1, then move the point to the
first @@ header, and thus C-c C-a would try to apply the same hunk again.