tests/test-check-pyflakes.t

branchcache: add a "pure topological head" fast path

In a narrow but actually quick common case, all topological heads are all on
the same branch and all open. In this case, computing the branch map is very
simple. We can quickly detect situation where this situation will not change.

So we update the V3 format to be able to express this situation and upgrade the
update code to detect we remains in that mode.

The branch cache is populated with the actual value when the branch map is
accessed, but the update_disk method can do the update without needing to
populate it.

#require test-repo pyflakes hg10

  $ . "$TESTDIR/helpers-testrepo.sh"

run pyflakes on all tracked files ending in .py or without a file ending
(skipping binary file random-seed)

  $ cat > test.py <<EOF
  > print(undefinedname)
  > EOF
  $ "$PYTHON" -m pyflakes test.py 2>/dev/null | "$TESTDIR/filterpyflakes.py"
  test.py:1:* undefined name 'undefinedname' (glob)
  
  $ cd "`dirname "$TESTDIR"`"

  $ testrepohg locate 'set:**.py or grep("^#!.*python")' \
  > -X hgext/fsmonitor/pywatchman \
  > -X contrib/python-zstandard \
  > -X mercurial/thirdparty \
  > 2>/dev/null \
  > | xargs "$PYTHON" -m pyflakes 2>/dev/null | "$TESTDIR/filterpyflakes.py"
  contrib/perf.py:*:* undefined name 'xrange' (glob) (?)
  mercurial/pycompat.py:*:* 'codecs' imported but unused (glob)
  mercurial/pycompat.py:*:* 'concurrent.futures' imported but unused (glob)
  mercurial/pycompat.py:*:* 'http.client as httplib' imported but unused (glob)
  mercurial/pycompat.py:*:* 'http.cookiejar as cookielib' imported but unused (glob)
  mercurial/pycompat.py:*:* 'io' imported but unused (glob)
  mercurial/pycompat.py:*:* 'queue' imported but unused (glob)
  mercurial/pycompat.py:*:* 'socketserver' imported but unused (glob)
  mercurial/pycompat.py:*:* 'xmlrpc.client as xmlrpclib' imported but unused (glob)
  mercurial/util.py:*:* 'pickle' imported but unused (glob)