stream-clone: disable gc for `_entries_walk` duration
The number of small container created turn Python in a gc-frenzy that seriously
impact performance.
This significantly boost performance. The following number comes from a large
private repository using perf::stream-locked-section:
base-line: 35.04 seconds
prev-change: 24.51 seconds (-30%)
this-change: 20.88 seconds (-40% from baseline; -15% from previous changes)
Test issue2761
$ hg init repo
$ cd repo
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1