Merge branch 'ks/ref-filter-sort-numerically'

"git for-each-ref --sort='contents:size'" sorts the refs according
to size numerically, giving a ref that points at a blob twelve-byte
(12) long before showing a blob hundred-byte (100) long.

* ks/ref-filter-sort-numerically:
  ref-filter: sort numerically when ":size" is used

diff --cc ref-filter.c
Simple merge

diff --cc t/t6300-for-each-ref.sh
Simple merge