ref-filter: sort numerically when ":size" is used

ref-filter: sort numerically when ":size" is used

Atoms like "raw" and "contents" have a ":size" option which can be used
to know the size of the data. Since these atoms have the cmp_type
FIELD_STR, they are sorted alphabetically from 'a' to 'z' and '0' to
'9'. Meaning, even when the ":size" option is used and what we
ultimatlely have is numbers, we still sort alphabetically.

For example, consider the the following case in a repo

refname			contents:size		raw:size
=======			=============		========
refs/heads/branch1	1130			1210
refs/heads/master	300			410
refs/tags/v1.0		140			260

Sorting with "--format="%(refname) %(contents:size) --sort=contents:size"
would give

refs/heads/branch1 1130
refs/tags/v1.0.0 140
refs/heads/master 300

which is an alphabetic sort, while what one might really expect is

refs/tags/v1.0.0 140
refs/heads/master 300
refs/heads/branch1 1130

which is a numeric sort (that is, a "$ sort -n file" as opposed to a
"$ sort file", where "file" contains only the "contents:size" or
"raw:size" info, each of which is on a newline).

Same is the case with "--sort=raw:size".

So, sort numerically whenever the sort is done with "contents:size" or
"raw:size" and do it the normal alphabetic way when "contents" or "raw"
are used with some other option (they are FIELD_STR anyways).

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Kousik Sanagavarapu <five231003@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>