[WIP PATCH] svnrdump dump --incremental

Here's a stab at implementing 'svnrdump dump --incremental'.

It does what 'svn checkout' does to get the first revision --- namely,
calls svn_ra_do_update2() with start_empty=TRUE.

TODO: it passes send_copyfrom_args=TRUE to svn_ra_do_update2(), so it
still has to go by itself and requests fulltexts (or, copyfrom fulltexts
and txdeltas against those) for files that have been copied. Currently,
for a cp-with-changes, it dumps nothing (neither fulltext nor delta).

TODO: dump revprops for the first revision. (haven't tested this)

Daniel