vcs-svn: improve support for reading large files

vcs-svn: improve support for reading large files

Move from uint32_t to off_t as the fundamental unit of length used by
the line_buffer library.  Performance would get worse if anything but
I think it's worth it for support of deltas that need to skip large
pieces (> 4 GiB).

Exception: buffer_read_string still takes a uint32_t, since it keeps
its result in an in-core obj_pool.

Callers still have to be updated to take advantage of this.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: David Barr <david.barr@cordelta.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>