AI 146111: update webkitmerge to handle git, svg, fix bugs

AI 146111: update webkitmerge to handle git, svg, fix bugs
  //branches/master/android/external/webkit/WebCore/Android.derived.mk
  add exclusion rules for svg
  //branches/master/android/external/webkit/WebCore/Android.mk
  add exclusion exceptions (e.g., files from /skia/ that we use)
  remove the need to have '\' at the end of each file
  //branches/master/android/external/webkit/WebKitTools/android/webkitmerge/webkitmerge.cpp
  don't delete modified files, but mark them with MANUAL_MERGE_REQUIRED
  add --execute option (in progress)
  add --perforce, --git options
  fix double add of file when it appears in sandbox and new webkit
  BUG=1553405

Automated import of CL 146111