merge-one-file: fix "expr: non-numeric argument"

When invoking expr to compare two numbers, don't quote the
variables which are the output of 'wc -c'. On OS X, this output
includes spaces, which expr balks at:

  $ sz0=`wc -c </etc/passwd`
  $ sz1=`wc -c </etc/passwd`
  $ echo "'$sz0'"
  '    3667'

  $ expr "$sz0" \< "$sz1" \* 2
  expr: non-numeric argument

  $ expr $sz0 \< $sz1 \* 2
  1

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>