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

@@ -1,79 +1,79 b''
 # A simple script for opening merge conflicts in editor
 # A loose translation of contrib/editmerge to powershell
 # Please make sure that both editmergeps.bat and editmerge.ps1 are available
 # via %PATH% and use the following Mercurial settings to enable it
 #
 # [ui]
 # editmergeps
 # editmergeps.args=$output
 # editmergeps.check=changed
 # editmergeps.premerge=keep
 
 $file=$args[0]
 
 function Get-Lines
 {
   Select-String "^<<<<<<" $file | % {"$($_.LineNumber)"}
 }
 
 $ed = $Env:HGEDITOR;
 if ($ed -eq $nil)
 {
   $ed = $Env:VISUAL;
 }
 if ($ed -eq $nil)
 {
   $ed = $Env:EDITOR;
 }
 if ($ed -eq $nil)
 {
   $ed = $(hg showconfig ui.editor);
 }
 if ($ed -eq $nil)
 {
   Write-Error "merge failed - unable to find editor"
   exit 1
 }
 
 if (($ed -eq "vim") -or ($ed -eq "emacs") -or `
     ($ed -eq "nano") -or ($ed -eq "notepad++"))
 {
   $lines = Get-Lines
   $firstline = if ($lines.Length -gt 0) { $lines[0] } else { $nil }
   $previousline = $nil;
 
 
   # open the editor to the first conflict until there are no more
   # or the user stops editing the file
   while (($firstline -ne $nil) -and ($firstline -ne $previousline))
   {
     if ($ed -eq "notepad++")
     {
         $linearg = "-n$firstline"
     }
     else
     {
         $linearg = "+$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 }
     Write-Output "firstline is: $firstline, previousline is: $previousline"
   }
 }
 else
 {
   $ "$ed" $file
 }
 
 $conflicts=Get-Lines
 if ($conflicts.Length -ne 0)
 {
   Write-Output "merge failed - resolve the conflicts (line $conflicts) then use 'hg resolve --mark'"
   exit 1
 }
 
 exit 0