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.

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