git-gui: rework handling of diff header

git-gui: rework handling of diff header

The fix in ca53c3f (Fix diff parsing for lines starting with "--" or "++",
2008-09-05) got a bug report from Johannes Sixt, that new files in the
index now looks like:

  new file mode 100644
  --- /dev/null
  +++ b/foo
  @@ -0,0 +1 @@
  +foo

The introduced problem was that the 'in-diff-header'-flag was unconditially
disabled. Now it is only disabled when a hunk line is detected. And also
re-enabled when we encounter a new diff header.

The second part solves also the issue reported by me for diffs with file
type changes (i.e. the ''error: Unhandled 2 way diff marker: {d}"', which
comes from the second 'diff --git' line).

Reported-by: Johannes Sixt <j.sixt@viscovery.net>
Reported-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>