git-gui i18n: mark strings for translation

Mark strings for translation in lib/index.tcl that were seemingly
left behind by 700e560 ("git-gui: Mark forgotten strings for
translation.", 2008-09-04) which marks string in do_revert_selection
procedure.
These strings are passed to unstage_help and add_helper procedures.

Signed-off-by: Vasco Almeida <vascomalmeida@sapo.pt>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: vascool

lib/index.tcl

@@ -291,7 +291,7 @@ proc do_unstage_selection {} {
 
 	if {[array size selected_paths] > 0} {
 		unstage_helper \
-			{Unstaging selected files from commit} \
+			[mc "Unstaging selected files from commit"] \
 			[array names selected_paths]
 	} elseif {$current_diff_path ne {}} {
 		unstage_helper \
@@ -343,7 +343,7 @@ proc do_add_selection {} {
 
 	if {[array size selected_paths] > 0} {
 		add_helper \
-			{Adding selected files} \
+			[mc "Adding selected files"] \
 			[array names selected_paths]
 	} elseif {$current_diff_path ne {}} {
 		add_helper \
@@ -385,7 +385,7 @@ proc do_add_all {} {
 			set paths [concat $paths $untracked_paths]
 		}
 	}
-	add_helper {Adding all changed files} $paths
+	add_helper [mc "Adding all changed files"] $paths
 }
 
 proc revert_helper {txt paths} {