improve subwcrev to not touch svnrev.cs files if theres no change; improves typical build times substantially when working on multiclient.

SubWCRev.bat

@@ -1,10 +1,34 @@
 @echo off
 
-rem first, generate one with sed using no revision number, in case svn isnt available
-"%~1..\sed.exe" s/\$WCREV\$/0/ < "%~1properties\svnrev_template" > "%~1properties\svnrev.cs"
+set TEMPFILE="%TEMP%-%RANDOM%-%RANDOM%-%RANDOM%-%RANDOM%"
+set SVNREV="%~1properties\svnrev.cs"
 
-rem try generating one from svn now. this will fail if svn is nonexistent, so...
-"%~1..\SubWCRev.exe" "%~1\.." "%~1properties\svnrev_template" "%~1properties\svnrev.cs"
+rem generate a wvnrev with sed using no revision number, in case svn isnt available
+@"%~1..\sed.exe" s/\$WCREV\$/0/ < "%~1properties\svnrev_template" > %TEMPFILE%
+
+rem try generating svnrev from svn now. this will fail if svn is nonexistent, so...
+@"%~1..\SubWCRev.exe" "%~1\.." "%~1properties\svnrev_template" %TEMPFILE% > nul
 
 rem ... ignore the error
 SET ERRORLEVEL=0
+
+rem if we didnt even have a svnrev, then go ahead and copy it
+if not exist %SVNREV% (
+   @copy /y %TEMPFILE% %SVNREV%
+) else if exist %TEMPFILE% (
+  rem check to see whether its any different, so we dont touch unchanged files
+  fc /b %TEMPFILE% %SVNREV% > nul
+  if %ERRORLEVEL% neq 0 (
+    echo Updated svnrev file
+	@copy /y %TEMPFILE% %SVNREV%
+  ) else (
+    echo Not touching unchanged svnrev file
+  )
+) else (
+  echo Ran into a weird error writing subwcrev output to tempfile: %TEMPFILE%
+)
+
+del %TEMPFILE%
+
+rem always let build proceed
+SET ERRORLEVEL=0