In 'svnmover', allow a couple of nested-branching functions to operate on a non-flat branch hierarchy without raising an assertion failure.
In 'svnmover', allow a couple of nested-branching functions to operate on a
non-flat branch hierarchy without raising an assertion failure.

The specific case this enables is where a branch-root element points to a
nested branch that doesn't currently exist. No test currently requires this,
but it makes the functions more widely applicable for future use.

* subversion/libsvn_delta/branch_nested.c
  (svn_branch__get_immediate_subbranch_eids): New, extracted...
  (svn_branch__get_immediate_subbranches): ... from here.
  (svn_branch__get_subtree,
   svn_branch__find_nested_branch_element_by_relpath): Use it, and ignore
    any subbranch-root element that points to a nonexistent nested branch
    instead of raising an assertion failure.


git-svn-id: f8a4e5e023278da1e04e203c7fe051e3c4285d88@1717949 13f79535-47bb-0310-9956-ffa450edef68

YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion,YueLinHo/Subversion