git-gui: allow undoing last revert

Accidental clicks on the revert hunk/lines buttons can cause loss of
work, and can be frustrating. So, allow undoing the last revert.

Right now, a stack or deque are not being used for the sake of
simplicity, so only one undo is possible. Any reverts before the
previous one are lost.

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

git-gui.sh

@@ -1350,6 +1350,8 @@ set is_submodule_diff 0
 set is_conflict_diff 0
 set selected_commit_type new
 set diff_empty_count 0
+set last_revert {}
+set last_revert_enc {}
 
 set nullid "0000000000000000000000000000000000000000"
 set nullid2 "0000000000000000000000000000000000000001"
@@ -3601,6 +3603,11 @@ $ctxm add command \
 	-command {apply_or_revert_range_or_line $cursorX $cursorY 1; do_rescan}
 set ui_diff_revertline [$ctxm index last]
 lappend diff_actions [list $ctxm entryconf $ui_diff_revertline -state]
+$ctxm add command \
+	-label [mc "Undo Last Revert"] \
+	-command {undo_last_revert; do_rescan}
+set ui_diff_undorevert [$ctxm index last]
+lappend diff_actions [list $ctxm entryconf $ui_diff_undorevert -state]
 $ctxm add separator
 $ctxm add command \
 	-label [mc "Show Less Context"] \
@@ -3680,7 +3687,7 @@ proc has_textconv {path} {
 }
 
 proc popup_diff_menu {ctxm ctxmmg ctxmsm x y X Y} {
-	global current_diff_path file_states
+	global current_diff_path file_states last_revert
 	set ::cursorX $x
 	set ::cursorY $y
 	if {[info exists file_states($current_diff_path)]} {
@@ -3694,6 +3701,7 @@ proc popup_diff_menu {ctxm ctxmmg ctxmsm x y X Y} {
 		tk_popup $ctxmsm $X $Y
 	} else {
 		set has_range [expr {[$::ui_diff tag nextrange sel 0.0] != {}}]
+		set u [mc "Undo Last Revert"]
 		if {$::ui_index eq $::current_diff_side} {
 			set l [mc "Unstage Hunk From Commit"]
 			set h [mc "Revert Hunk"]
@@ -3739,12 +3747,20 @@ proc popup_diff_menu {ctxm ctxmmg ctxmsm x y X Y} {
 			}
 		}
 
+		if {$last_revert eq {}} {
+			set undo_state disabled
+		} else {
+			set undo_state normal
+		}
+
 		$ctxm entryconf $::ui_diff_applyhunk -state $s -label $l
 		$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
+		$ctxm entryconf $::ui_diff_undorevert -state $undo_state \
+			-label $u
 
 		tk_popup $ctxm $X $Y
 	}

lib/diff.tcl

@@ -569,7 +569,7 @@ proc read_diff {fd conflict_size cont_info} {
 
 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
+	global ui_diff ui_index file_states last_revert last_revert_enc
 
 	if {$current_diff_path eq {} || $current_diff_header eq {}} return
 	if {![lock_index apply_hunk]} return
@@ -610,18 +610,25 @@ proc apply_or_revert_hunk {x y revert} {
 		set e_lno end
 	}
 
+	set wholepatch "$current_diff_header[$ui_diff get $s_lno $e_lno]"
+
 	if {[catch {
 		set enc [get_path_encoding $current_diff_path]
 		set p [eval git_write $apply_cmd]
 		fconfigure $p -translation binary -encoding $enc
-		puts -nonewline $p $current_diff_header
-		puts -nonewline $p [$ui_diff get $s_lno $e_lno]
+		puts -nonewline $p $wholepatch
 		close $p} err]} {
 		error_popup "$failed_msg\n\n$err"
 		unlock_index
 		return
 	}
 
+	if {$revert} {
+		# Save a copy of this patch for undoing reverts.
+		set last_revert $wholepatch
+		set last_revert_enc $enc
+	}
+
 	$ui_diff conf -state normal
 	$ui_diff delete $s_lno $e_lno
 	$ui_diff conf -state disabled
@@ -653,7 +660,7 @@ proc apply_or_revert_hunk {x y revert} {
 
 proc apply_or_revert_range_or_line {x y revert} {
 	global current_diff_path current_diff_header current_diff_side
-	global ui_diff ui_index file_states
+	global ui_diff ui_index file_states last_revert
 
 	set selected [$ui_diff tag nextrange sel 0.0]
 
@@ -852,5 +859,43 @@ proc apply_or_revert_range_or_line {x y revert} {
 		return
 	}
 
+	if {$revert} {
+		# Save a copy of this patch for undoing reverts.
+		set last_revert $current_diff_header$wholepatch
+		set last_revert_enc $enc
+	}
+
+	unlock_index
+}
+
+# Undo the last line/hunk reverted. When hunks and lines are reverted, a copy
+# of the diff applied is saved. Re-apply that diff to undo the revert.
+#
+# Right now, we only use a single variable to hold the copy, and not a
+# stack/deque for simplicity, so multiple undos are not possible. Maybe this
+# can be added if the need for something like this is felt in the future.
+proc undo_last_revert {} {
+	global last_revert current_diff_path current_diff_header
+	global last_revert_enc
+
+	if {$last_revert eq {}} return
+	if {![lock_index apply_hunk]} return
+
+	set apply_cmd {apply --whitespace=nowarn}
+	set failed_msg [mc "Failed to undo last revert."]
+
+	if {[catch {
+		set enc $last_revert_enc
+		set p [eval git_write $apply_cmd]
+		fconfigure $p -translation binary -encoding $enc
+		puts -nonewline $p $last_revert
+		close $p} err]} {
+		error_popup "$failed_msg\n\n$err"
+		unlock_index
+		return
+	}
+
+	set last_revert {}
+
 	unlock_index
 }