Fix rebar.bat path handling

Use "%~f0" to get the full path of rebat.bat to correctly locate the
rebar script. Also put the script name in quotes when passing it to
escript.exe to correctly handle paths with spaces.

rebar.bat

@@ -1,4 +1,4 @@
 @echo off
 setlocal
-set rebarscript=%0
-escript.exe %rebarscript:.bat=% %*
+set rebarscript=%~f0
+escript.exe "%rebarscript:.bat=%" %*