contrib: run editmergeps.ps1 from the same location as editmergeps.bat

This change is needed for cases when user does not put editmergeps.bat
directly into PATH, but rather uses 'merge-tools.editmergeps.executable'
config option to provide a full path to editmergeps.bat. In such cases,
editmergeps.ps1 cannot be run simply by name, it needs a full path. In
BATCH file %~dp0 stands for the directory in which the original file
is located.

contrib/editmergeps.bat

@@ -1,2 +1,2 b''
 @echo off
-powershell -NoProfile -ExecutionPolicy unrestricted -Command "& 'editmergeps.ps1' %*"
+powershell -NoProfile -ExecutionPolicy unrestricted -Command "& '%~dp0\editmergeps.ps1' %*"