remotefilelog: replace a "a=[expr]; b=a[0]" by "b=expr; a = [b]"

Authored by martinvonz.

Description

remotefilelog: replace a "a=[expr]; b=a[0]" by "b=expr; a = [b]"

Simpler is better.

Differential Revision: https://phab.mercurial-scm.org/D5384