[RFC/PATCH] gitk: Visualize a merge commit with a right-click in gitk

[Jason Holden <jason.k.holden.swdev@gmail.com>]

When first doing a merge in git-gui, the "Visualize Merge" button is
quite helpful to visualize the changes due to a merge.
But once the merge is complete, there's not a similarly convenient
way to recreate that merge view in gitk.

This commit adds to gitk the ability to right-click on a merge commit and
bring up a new gitk window displaying only those commits involved in
the merge.

When right-clicking on a non-merge commit, this option is grayed out.  This
patch also supports correct visualization of octopus merges

Signed-off-by: Jason Holden <jason.k.holden.swdev@gmail.com>
---
 gitk | 33 +++++++++++++++++++++++++++++++++
 1 file changed, 33 insertions(+)

diff --git a/gitk b/gitk
index 379582a..17e1fcb 100755
--- a/gitk
+++ b/gitk
@@ -2551,6 +2551,7 @@ proc makewindow {} {
 	{mc "Compare with marked commit" command compare_commits}
 	{mc "Diff this -> marked commit" command {diffvsmark 0}}
 	{mc "Diff marked commit -> this" command {diffvsmark 1}}
+	{mc "Visualize this merge" command visualize_merge}
     }
     $rowctxmenu configure -tearoff 0
 
@@ -2590,6 +2591,31 @@ proc makewindow {} {
     $diff_menu configure -tearoff 0
 }
 
+# Return the number of parents for a given sha1 id
+proc get_numparents_from_id {id} {
+    global parentlist
+    set row [rowofcommit $id]
+    return [llength [lindex $parentlist $row]]
+}
+
+proc visualize_merge {} {
+    global parents currentid parentlist
+    global rowmenuid
+
+    set num_parents [get_numparents_from_id $rowmenuid]
+    set row [rowofcommit $rowmenuid]
+
+    if {$num_parents >= 2} {
+	set revlist $rowmenuid
+	for { set i 1 } {$i < $num_parents} {incr i} {
+
+	    set revlist "$revlist [lindex $parentlist $row 0]..[lindex $parentlist $row $i] $rowmenuid"
+	}
+	
+	eval exec gitk $revlist
+    }
+}
+
 # Windows sends all mouse wheel events to the current focused window, not
 # the one where the mouse hovers, so bind those events here and redirect
 # to the correct window
@@ -8577,6 +8603,13 @@ proc rowmenu {x y id} {
 	$menu entryconfigure 9 -state $mstate
 	$menu entryconfigure 10 -state $mstate
 	$menu entryconfigure 11 -state $mstate
+
+	# Disable visualize-merge on only one parent
+	if {[get_numparents_from_id $id] == 1} {
+	    $menu entryconfigure 15 -state disabled
+	} else {
+	    $menu entryconfigure 15 -state normal
+	}
     } else {
 	set menu $fakerowmenu
     }
-- 
1.8.1.rc3.28.g0ab5d1f