git-gui: use "untracked" for files which are not known to git

"untracked" is the right phrase for files new to git. For example
git-status uses this phrase. Also make the question shorter.

Signed-off-by: Bert Wesarg <bert.wesarg@googlemail.com>
Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>

lib/index.tcl

@@ -356,20 +356,20 @@ proc do_add_all {} {
 	global file_states
 
 	set paths [list]
-	set unknown_paths [list]
+	set untracked_paths [list]
 	foreach path [array names file_states] {
 		switch -glob -- [lindex $file_states($path) 0] {
 		U? {continue}
 		?M -
 		?T -
 		?D {lappend paths $path}
-		?O {lappend unknown_paths $path}
+		?O {lappend untracked_paths $path}
 		}
 	}
-	if {[llength $unknown_paths]} {
-		set reply [ask_popup [mc "There are unknown files do you also want to stage those?"]]
+	if {[llength $untracked_paths]} {
+		set reply [ask_popup [mc "Stage also untracked files?"]]
 		if {$reply} {
-			set paths [concat $paths $unknown_paths]
+			set paths [concat $paths $untracked_paths]
 		}
 	}
 	add_helper {Adding all changed files} $paths