util: migrate fileset._sizetoint to util.sizetoint

The size counting code introduced in 2c4cd1c42365 duplicated existing
(but unknown-to-me) code in fileset, so prepare to eliminate the
duplication.
ui: use util.sizetoint in configbytes
Added tag 2.6.1 for changeset ddc7a6be2021

--HG--
branch : stable
Added signature for changeset ddc7a6be2021

--HG--
branch : stable
merge with stable