CTest.UpdateHG: Fix repo URL for leading slash

Use "file:///..." instead of "file:////..." when the file system path
starts in a slash.  Commit 0916cc88 (CTest.UpdateHG: Fix repo URL for
local filesystem, 2012-03-02) added a third slash after "file://"
unconditionally.  This worked for many file systems but not on Cygwin
where "file:////cygdrive/..." looks like "file://" followed by a network
file path "//cygdrive/...".  Add the slash only if the file system path
does not already start with one.

Tests/CTestUpdateHG.cmake.in

@@ -28,7 +28,10 @@ run_child(
   WORKING_DIRECTORY ${TOP}/repo.hg
   COMMAND ${HG} init
   )
-set(REPO file:///${TOP}/repo.hg)
+if(NOT "${TOP}" MATCHES "^/")
+  set(slash /)
+endif()
+set(REPO file://${slash}${TOP}/repo.hg)
 
 #-----------------------------------------------------------------------------
 # Import initial content into the repository.