load-fs-vtable.c

Fix the 'svnadmin load' part of issue #4476 "Mergeinfo containing r0 makes
svnsync and svnadmin dump fail".

Make 'svnadmin load' accept mergeinfo containing r0 (with a warning) if the
option to validate properties is not enabled. In that case that mergeinfo
property will not be adjusted for revision numbers or paths. Add a test.

* subversion/include/svn_repos.h
(svn_repos_notify_warning_t): Add a new value for invalid mergeinfo.

* subversion/libsvn_repos/load-fs-vtable.c
(adjust_mergeinfo_property): New, factored out of ...
(set_node_property): ... here. If adjusting the mergeinfo fails, return an
error if validating properties, or give a warning and use the invalid
value otherwise.

* subversion/tests/libsvn_repos/dump-load-test.c
(test_dump_bad_props): New, factored out of...
(test_dump_r0_mergeinfo): ... here.
(test_load_bad_props,
load_r0_mergeinfo_notifier,
test_load_r0_mergeinfo): New.
(test_funcs): Add the new test.