vcs-svn: allow 64-bit Prop-Content-Length

vcs-svn: allow 64-bit Prop-Content-Length

Currently the vcs-svn/ library only pays attention to the presence of
the Prop-Content-Length field and doesn't care about its value, but
some day we might care about the value.  Parse it as an off_t instead
of arbitrarily limiting to 32 bits for intuitiveness.

So now you can import from a dump with more than 2 GiB of properties
for a node.  In practice that isn't likely to happen often, and this
is mostly meant as a cleanup.

Based-on-patch-by: David Barr <davidbarr@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>