vfs: modernize the detection of the main thread
There weren't a lot of good choices when py27 was supported, but starting with
py34, `threading.main_thread()` is available. This gets us away from an
undocumented, internal symbol, and drops a pytype suppression statement. It is
also apparently no longer reliable after a process fork.[1][2]
[1] https://stackoverflow.com/a/
23207116
[2] https://github.com/python/cpython/blob/v3.6.3/Lib/threading.py#L1334
https://bz.mercurial-scm.org/1502
Initialize repository
$ hg init foo
$ touch foo/a && hg -R foo commit -A -m "added a"
adding a
$ hg clone foo foo1
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ echo "bar" > foo1/a && hg -R foo1 commit -m "edit a in foo1"
$ echo "hi" > foo/a && hg -R foo commit -m "edited a foo"
$ hg -R foo1 pull
pulling from $TESTTMP/foo
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files (+1 heads)
new changesets 273d008d6e8e
(run 'hg heads' to see heads, 'hg merge' to merge)
$ hg -R foo1 book branchy
$ hg -R foo1 book
* branchy 1:e3e522925eff
Pull. Bookmark should not jump to new head.
$ echo "there" >> foo/a && hg -R foo commit -m "edited a again"
$ hg -R foo1 pull
pulling from $TESTTMP/foo
searching for changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 1 changes to 1 files
new changesets 84a798d48b17
(run 'hg update' to get a working copy)
$ hg -R foo1 book
* branchy 1:e3e522925eff