Rename a minimum format constant for consistency with others.

* subversion/libsvn_fs_fs/fs.h
  (SVN_FS_FS__MIN_MERGEINFO_IN_CHANGES_FORMAT): Rename to ...
  (SVN_FS_FS__MIN_MERGEINFO_IN_CHANGED_FORMAT): ... this.

* subversion/libsvn_fs_fs/low_level.c
  (svn_fs_fs__write_changes): Update usage.

Suggested by: julianfoad

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1631185 13f79535-47bb-0310-9956-ffa450edef68