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.

--- a/contrib/editmergeps.ps1	Tue May 30 06:02:31 2017 -0700
+++ b/contrib/editmergeps.ps1	Tue May 30 06:17:51 2017 -0700
@@ -65,7 +65,7 @@
 }
 else
 {
-  $ "$ed" $file
+  & "$ed" $file
 }
 
 $conflicts=Get-Lines