contrib: make editmergeps use -NoNewWindow option in Start-Process cmdlet

Running 'Start-Process -Wait "vim" "+10" "filename"' from PowerShell
actually spawns a separate cmd window to run vim in. This looks ugly
and in most cases not what user wants. During my initial testing I was
using the Cmder app, which made me not notice this (it captures new
windows as new tabs).

contrib/editmergeps.ps1

@@ -56,7 +56,7 b' if (($ed -eq "vim") -or ($ed -eq "emacs"'
         $linearg = "+$firstline"
     }
 
-    Start-Process -Wait $ed $linearg,$file
+    Start-Process -Wait -NoNewWindow $ed $linearg,$file
     $previousline = $firstline
     $lines = Get-Lines
     $firstline = if ($lines.Length -gt 0) { $lines[0] } else { $nil }