Extend the mtcc tests to change a few files remotely and fix a few more
issues found using this testcase.

* subversion/libsvn_client/mtcc.c
  (commit_file): Use the stream read checksum. Explicitly close the used
    files in all cases or we won't get a checksum.

* subversion/tests/libsvn_client/mtcc-test.c
  (test_update_files): New test.
  (test_list): Add test_update_files.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1550831 13f79535-47bb-0310-9956-ffa450edef68