contrib: fix a bug preventing editmergeps.ps1 from running unknonw editors

'$ executable' is not a way to run executable in powershell, '& executable'
is instead. Found this when testing with regular Windows notepad.

contrib/editmergeps.ps1

@@ -65,7 +65,7 b' if (($ed -eq "vim") -or ($ed -eq "emacs"'
 }
 else
 {
-  $ "$ed" $file
+  & "$ed" $file
 }
 
 $conflicts=Get-Lines