git-gui: support for diff3 conflict style

This adds highlight support for the diff3 conflict style.

The common pre-image will be reversed to --, because it has been removed
and replaced with ours or theirs side respectively.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

git-gui.sh

@@ -3581,6 +3581,9 @@ $ui_diff tag conf d_s- \
 $ui_diff tag conf d< \
 	-foreground orange \
 	-font font_diffbold
+$ui_diff tag conf d| \
+	-foreground orange \
+	-font font_diffbold
 $ui_diff tag conf d= \
 	-foreground orange \
 	-font font_diffbold

lib/diff.tcl

@@ -347,6 +347,10 @@ proc start_show_diff {cont_info {add_opts {}}} {
 	}
 
 	set ::current_diff_inheader 1
+	# Detect pre-image lines of the diff3 conflict-style. They are just
+	# '++' lines which is not bijective. Thus, we need to maintain a state
+	# across lines.
+	set ::conflict_in_pre_image 0
 	fconfigure $fd \
 		-blocking 0 \
 		-encoding [get_path_encoding $path] \
@@ -449,11 +453,23 @@ proc read_diff {fd conflict_size cont_info} {
 			{--} {set tags d_--}
 			{++} {
 				set regexp [string map [list %conflict_size $conflict_size]\
-								{^\+\+([<>=]){%conflict_size}(?: |$)}]
+								{^\+\+([<>=|]){%conflict_size}(?: |$)}]
 				if {[regexp $regexp $line _g op]} {
 					set is_conflict_diff 1
 					set line [string replace $line 0 1 {  }]
 					set tags d$op
+
+					# The ||| conflict-marker marks the start of the pre-image.
+					# All those lines are also prefixed with '++'. Thus we need
+					# to maintain this state.
+					set ::conflict_in_pre_image [expr {$op eq {|}}]
+				} elseif {$::conflict_in_pre_image} {
+					# This is a pre-image line. It is the one which both sides
+					# are based on. As it has also the '++' line start, it is
+					# normally shown as 'added'. Invert this to '--' to make
+					# it a 'removed' line.
+					set line [string replace $line 0 1 {--}]
+					set tags d_--
 				} else {
 					set tags d_++
 				}