unshelve: create a matcher only if required on creating unshelve ctx
Differential Revision: https://phab.mercurial-scm.org/D6687
running: init test1
result: 0
running: add foo
result: 0
running: commit -m commit1 -d 2000-01-01 foo
result: 0
running: commit -m commit2 -d 2000-01-02 foo
result: 0
running: log -r 0
changeset: 0:0e4634943879
user: test
date: Sat Jan 01 00:00:00 2000 +0000
summary: commit1
result: 0
running: log -r tip
changeset: 1:45589e459b2e
tag: tip
user: test
date: Sun Jan 02 00:00:00 2000 +0000
summary: commit2
result: 0