manifest: change terminology for storage in context classes
Storage may not be backed by revlogs. So it is wrong to refer
to storage as "revlog" or use abbreviations like "rl."
This commit replaces revlog terminology in manifest context classes
and starts referring to things as "store" or "storage."
Differential Revision: https://phab.mercurial-scm.org/D4389
$ hg init
$ echo 123 > a
$ hg add a
$ hg commit -m "first" a
$ mkdir sub
$ echo 321 > sub/b
$ hg add sub/b
$ hg commit -m "second" sub/b
$ cat sub/b
321
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cat sub/b 2>/dev/null || echo "sub/b not present"
sub/b not present
$ test -d sub || echo "sub not present"
sub not present