Use a simpler and more reliable command for converting from HEAD to
commit-ish. Funny thing, they have a command designed for this. ;]

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@146325 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/cmake/modules/VersionFromVCS.cmake b/cmake/modules/VersionFromVCS.cmake
index bf57763..a55e9b8 100644
--- a/cmake/modules/VersionFromVCS.cmake
+++ b/cmake/modules/VersionFromVCS.cmake
@@ -50,7 +50,7 @@
         set(git_svn_rev "")
       endif()
       execute_process(COMMAND
-                      ${git_executable} show-ref --abbrev --hash --head HEAD
+                      ${git_executable} rev-parse --short HEAD
                       WORKING_DIRECTORY ${CMAKE_CURRENT_SOURCE_DIR}
                       TIMEOUT 5
                       RESULT_VARIABLE git_result