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>

lib/diff.tcl

@@ -382,23 +382,35 @@ proc read_diff {fd conflict_size cont_info} {
 		foreach {line markup} [parse_color_line $line] break
 		set line [string map {\033 ^} $line]
 
-		# -- Cleanup uninteresting diff header lines.
+		# -- Check for start of diff header.
+		if {   [string match {diff --git *}      $line]
+		    || [string match {diff --cc *}       $line]
+		    || [string match {diff --combined *} $line]} {
+			set ::current_diff_inheader 1
+		}
+
+		# -- Check for end of diff header (any hunk line will do this).
 		#
+		if {[regexp {^@@+ } $line]} {set ::current_diff_inheader 0}
+
 		if {$::current_diff_inheader} {
+			append current_diff_header $line "\n"
+
+			# -- Cleanup uninteresting diff header lines.
+			#
 			if {   [string match {diff --git *}      $line]
 			    || [string match {diff --cc *}       $line]
 			    || [string match {diff --combined *} $line]
 			    || [string match {--- *}             $line]
 			    || [string match {+++ *}             $line]} {
-				append current_diff_header $line "\n"
 				continue
 			}
 		}
+
 		if {[string match {index *} $line]} continue
 		if {$line eq {deleted file mode 120000}} {
 			set line "deleted symlink"
 		}
-		set ::current_diff_inheader 0
 
 		# -- Automatically detect if this is a 3 way diff.
 		#