[sparse-directories] Depth parameter for the editor API

Karl, attached is a patch attempting to implement the outstanding
suggestions from Peter's "Default depth in reporter?" review mail:

 http://subversion.tigris.org/servlets/ReadMsg?list=dev&msgNo=124363

I've dropped the DEPTH parameter from the svn_repos_begin_report3()
API and its callers, in favor of writing the DEPTH passed to
set_path() to the report, and subsequently using that depth during the
editor drive. An unknown value passed to write_path_info() will
default to report_baton_t->default_depth (a field which I renamed from
depth). I've got svn_repos_begin_report3() setting
report_baton_t->default_depth to svn_depth_infinity, but perhaps it
should be smuggling in a depth of svn_depth_unknown?

You'll notice that I had to change svn_wc_crawl_revisions3() to use
the passed-in DEPTH parameter (when known) for the first call to
set_path() (there's another call to set_path() near the beginning of
this routine which may need similar treatment).