vcs-svn: allow import of > 4GiB files

vcs-svn: allow import of > 4GiB files

There is no reason in principle that an svn-format dump would not
be able to represent a file whose length does not fit in a 32-bit
integer.  Use off_t consistently (instead of uint32_t) to represent
file lengths so we can handle that.

Most of our code is already ready to do that without this patch and
already passes values of type off_t around.  The type mismatch due to
stragglers was noticed with gcc -Wtype-limits.

Inspired-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>