git-gui: allow reverting selected hunk

Just like the user can select a hunk to stage or unstage, add the
ability to revert hunks.

Signed-off-by: Pratyush Yadav <me@yadavpratyush.com>

git-gui.sh

@@ -3582,7 +3582,7 @@ set ctxm .vpane.lower.diff.body.ctxm
 menu $ctxm -tearoff 0
 $ctxm add command \
 	-label [mc "Apply/Reverse Hunk"] \
-	-command {apply_hunk $cursorX $cursorY}
+	-command {apply_or_revert_hunk $cursorX $cursorY 0}
 set ui_diff_applyhunk [$ctxm index last]
 lappend diff_actions [list $ctxm entryconf $ui_diff_applyhunk -state]
 $ctxm add command \
@@ -3591,6 +3591,11 @@ $ctxm add command \
 set ui_diff_applyline [$ctxm index last]
 lappend diff_actions [list $ctxm entryconf $ui_diff_applyline -state]
 $ctxm add separator
+$ctxm add command \
+	-label [mc "Revert Hunk"] \
+	-command {apply_or_revert_hunk $cursorX $cursorY 1}
+set ui_diff_reverthunk [$ctxm index last]
+lappend diff_actions [list $ctxm entryconf $ui_diff_reverthunk -state]
 $ctxm add command \
 	-label [mc "Revert Line"] \
 	-command {apply_or_revert_range_or_line $cursorX $cursorY 1; do_rescan}
@@ -3691,6 +3696,8 @@ proc popup_diff_menu {ctxm ctxmmg ctxmsm x y X Y} {
 		set has_range [expr {[$::ui_diff tag nextrange sel 0.0] != {}}]
 		if {$::ui_index eq $::current_diff_side} {
 			set l [mc "Unstage Hunk From Commit"]
+			set h [mc "Revert Hunk"]
+
 			if {$has_range} {
 				set t [mc "Unstage Lines From Commit"]
 				set r [mc "Revert Lines"]
@@ -3700,6 +3707,8 @@ proc popup_diff_menu {ctxm ctxmmg ctxmsm x y X Y} {
 			}
 		} else {
 			set l [mc "Stage Hunk For Commit"]
+			set h [mc "Revert Hunk"]
+
 			if {$has_range} {
 				set t [mc "Stage Lines For Commit"]
 				set r [mc "Revert Lines"]
@@ -3734,6 +3743,9 @@ proc popup_diff_menu {ctxm ctxmmg ctxmsm x y X Y} {
 		$ctxm entryconf $::ui_diff_applyline -state $s -label $t
 		$ctxm entryconf $::ui_diff_revertline -state $revert_state \
 			-label $r
+		$ctxm entryconf $::ui_diff_reverthunk -state $revert_state \
+			-label $h
+
 		tk_popup $ctxm $X $Y
 	}
 }

lib/diff.tcl

@@ -567,24 +567,31 @@ proc read_diff {fd conflict_size cont_info} {
 	}
 }
 
-proc apply_hunk {x y} {
+proc apply_or_revert_hunk {x y revert} {
 	global current_diff_path current_diff_header current_diff_side
 	global ui_diff ui_index file_states
 
 	if {$current_diff_path eq {} || $current_diff_header eq {}} return
 	if {![lock_index apply_hunk]} return
 
-	set apply_cmd {apply --cached --whitespace=nowarn}
+	set apply_cmd {apply --whitespace=nowarn}
 	set mi [lindex $file_states($current_diff_path) 0]
 	if {$current_diff_side eq $ui_index} {
 		set failed_msg [mc "Failed to unstage selected hunk."]
-		lappend apply_cmd --reverse
+		lappend apply_cmd --reverse --cached
 		if {[string index $mi 0] ne {M}} {
 			unlock_index
 			return
 		}
 	} else {
-		set failed_msg [mc "Failed to stage selected hunk."]
+		if {$revert} {
+			set failed_msg [mc "Failed to revert selected hunk."]
+			lappend apply_cmd --reverse
+		} else {
+			set failed_msg [mc "Failed to stage selected hunk."]
+			lappend apply_cmd --cached
+		}
+
 		if {[string index $mi 1] ne {M}} {
 			unlock_index
 			return
@@ -619,13 +626,17 @@ proc apply_hunk {x y} {
 	$ui_diff delete $s_lno $e_lno
 	$ui_diff conf -state disabled
 
+	# Check if the hunk was the last one in the file.
 	if {[$ui_diff get 1.0 end] eq "\n"} {
 		set o _
 	} else {
 		set o ?
 	}
 
-	if {$current_diff_side eq $ui_index} {
+	# Update the status flags.
+	if {$revert} {
+		set mi [string index $mi 0]$o
+	} elseif {$current_diff_side eq $ui_index} {
 		set mi ${o}M
 	} elseif {[string index $mi 0] eq {_}} {
 		set mi M$o