branchcache: do not accept "empty update"
This currently does not happens and it will be simpler that is remains that way.
If all update do something, we will be able to simply declare, in a later
changesets, that all update to result in a dirty branchcache.
#require test-repo pure
$ . "$TESTDIR/helpers-testrepo.sh"
$ cd "$TESTDIR"/..
$ testrepohg files 'set:(**.py) - grep(pygments)' \
> -X hgext/fsmonitor/pywatchman \
> -X mercurial/cffi \
> -X mercurial/thirdparty \
> | sed 's|\\|/|g' | xargs "$PYTHON" contrib/check-py3-compat.py \
> | sed 's/[0-9][0-9]*)$/*)/'
hgext/convert/transport.py: error importing: <*Error> No module named 'svn.client' (error at transport.py:*) (glob) (?)
mercurial/scmwindows.py: error importing: <ValueError> _type_ 'v' not supported (error at win32.py:*) (no-windows !)
mercurial/win32.py: error importing: <ValueError> _type_ 'v' not supported (error at win32.py:*) (no-windows !)
mercurial/windows.py: error importing: <*Error> No module named 'msvcrt' (error at windows.py:*) (glob) (no-windows !)
mercurial/posix.py: error importing: <*Error> No module named 'fcntl' (error at posix.py:*) (glob) (windows !)
mercurial/scmposix.py: error importing: <*Error> No module named 'fcntl' (error at scmposix.py:*) (glob) (windows !)
#if pygments
$ testrepohg files 'set:(**.py) and grep(pygments)' | sed 's|\\|/|g' \
> | xargs "$PYTHON" contrib/check-py3-compat.py \
> | sed 's/[0-9][0-9]*)$/*)/'
#endif