Fix expansion of custom keywords with values that contain '='.
* subversion/libsvn_subr/subst.c (build_keywords): Don't use svn_cstring_split() to find the '=' separator. It is not a suitable API for this purpose. Instead, scan for the first occurrence of '=' and split keyword name from custom format there.
* subversion/tests/libsvn_subr/subst_translate-test.c (test_svn_subst_build_keywords3): Expand to cover a custom keyword with '='. Also assert that a non-NULL expanded keyword value is returned to avoid crashing this test with a NULL-deref.