cvsexportcommit: fix massive commits

cvsexportcommit: fix massive commits

Because we feed the changed filenames to CVS on the command
line, it was possible for massive commits to overflow the
system exec limits. Instead, we now do an xargs-like split
of the arguments.

This means that we lose some of the atomicity of calling CVS
in one shot. Since CVS commits are not atomic, but the CVS
protocol is, the possible effects of this are not clear;
however, since CVS doesn't provide a different interface,
this is our only option for large commits (short of writing
a CVS client library).

The argument size limit is arbitrarily set to 64kB. This
should be high enough to trigger only in rare cases where it
is necessary, so normal-sized commits are not affected by
the atomicity change.

Signed-off-by: Junio C Hamano <gitster@pobox.com>