Fix issue #4132 'merge of replaced source asserts'.

* subversion/libsvn_client/merge.c

  (find_gaps_in_merge_source_history): Return early if there can't possibly
   be a gap.

* subversion/tests/cmdline/merge_tests.py

  (svnmucc_abuse_1): Remove XFail decorator, adjust comment re failure
   status.

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1453425 13f79535-47bb-0310-9956-ffa450edef68