git-gui: Support passing blame to a parent commit.

Add a context menu item that switches the view to the
parent of the commit under cursor. It is useful to see
how the file looked before the change, and find older
changes in the same lines.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/blame.tcl

@@ -262,6 +262,9 @@ constructor new {i_commit i_path} {
 	$w.ctxm add command \
 		-label [mc "Show History Context"] \
 		-command [cb _gitkcommit]
+	$w.ctxm add command \
+		-label [mc "Blame Parent Commit"] \
+		-command [cb _blameparent]
 
 	foreach i $w_columns {
 		for {set g 0} {$g < [llength $group_colors]} {incr g} {
@@ -790,19 +793,27 @@ method _load_commit {cur_w cur_d pos} {
 	set lno [lindex [split [$cur_w index $pos] .] 0]
 	set dat [lindex $line_data $lno]
 	if {$dat ne {}} {
-		lappend history [list \
-			$commit $path \
-			$highlight_column \
-			$highlight_line \
-			[lindex [$w_file xview] 0] \
-			[lindex [$w_file yview] 0] \
-			]
-		set commit [lindex $dat 0]
-		set path   [lindex $dat 1]
-		_load $this [list [lindex $dat 2]]
+		_load_new_commit $this  \
+			[lindex $dat 0] \
+			[lindex $dat 1] \
+			[list [lindex $dat 2]]
 	}
 }
 
+method _load_new_commit {new_commit new_path jump} {
+	lappend history [list \
+		$commit $path \
+		$highlight_column \
+		$highlight_line \
+		[lindex [$w_file xview] 0] \
+		[lindex [$w_file yview] 0] \
+		]
+
+	set commit $new_commit
+	set path   $new_path
+	_load $this $jump
+}
+
 method _showcommit {cur_w lno} {
 	global repo_config
 	variable active_color
@@ -969,6 +980,23 @@ method _gitkcommit {} {
 	}
 }
 
+method _blameparent {} {
+	set dat [_get_click_amov_info $this]
+	if {$dat ne {}} {
+		set cmit [lindex $dat 0]
+
+		if {[catch {set cparent [git rev-parse --verify "$cmit^"]}]} {
+			error_popup [strcat [mc "Cannot find parent commit:"] "\n\n$err"]
+			return;
+		}
+
+		_load_new_commit $this  \
+			$cparent \
+			[lindex $dat 1] \
+			[list [lindex $dat 2]]
+	}
+}
+
 method _show_tooltip {cur_w pos} {
 	if {$tooltip_wm ne {}} {
 		_open_tooltip $this $cur_w