vcs-svn: suppress -Wtype-limits warning

vcs-svn: suppress -Wtype-limits warning

On 32-bit architectures with 64-bit file offsets, gcc 4.3 and earlier
produce the following warning:

	    CC vcs-svn/sliding_window.o
	vcs-svn/sliding_window.c: In function `check_overflow':
	vcs-svn/sliding_window.c:36: warning: comparison is always false \
	    due to limited range of data type

The warning appears even when gcc is run without any warning flags
(PR12963).  In later versions it can be reproduced with -Wtype-limits,
which is implied by -Wextra.

On 64-bit architectures it really is possible for a size_t not to be
representable as an off_t so the check being warned about is not
actually redundant.  But even false positives are distracting.  Avoid
the warning by making the "len" argument to check_overflow a
uintmax_t; no functional change intended.

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