Show the program version in the Help header.

1. On startup read the file git-rev if exists and initialize a new global
G_program_version.
2. In filechooser's help page's header show the version.
This helps to easily identify what version the user is running without
asking him to cat git-rev file manually.

filechooser.lua

@@ -498,7 +498,8 @@ function FileChooser:addAllCommands()
 	self.commands:add(KEY_H,nil,"H",
 		"show help page",
 		function(self)
-			HelpPage:show(0, G_height, self.commands)
+			local title="Hotkeys  "..G_program_version
+			HelpPage:show(0, G_height, self.commands, title)
 			self.pagedirty = true
 		end
 	)

reader.lua

@@ -99,6 +99,15 @@ if optarg["G"] ~= nil then
 	globalgamma = optarg["G"]
 end
 
+local vfile = io.open("git-rev", "r")
+if vfile then
+	G_program_version = vfile:read("*a") or "?"
+	G_program_version = G_program_version:gsub("[\n\r]+", "")
+	vfile.close()
+else
+	G_program_version = "(unknown version)"
+end
+
 if util.isEmulated()==1 then
 	input.open("")
 	-- SDL key codes