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 to represent file lengths (in place
of using uint32_t in some contexts) so we can handle that.

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

While at it, tighten the parsing of the Text-content-length field to
make sure it is a number and does not overflow, and tighten other
overflow checks as that value is passed around and manipulated.

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