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 no-windows serve
$ . "$TESTDIR/remotefilelog-library.sh"
$ cat >> $HGRCPATH <<EOF
> [extensions]
> remotefilelog=
> share=
> EOF
$ hg init master
$ cd master
$ cat >> .hg/hgrc <<EOF
> [remotefilelog]
> server=True
> EOF
$ echo x > x
$ hg commit -qAm x
$ cd ..
$ hgcloneshallow ssh://user@dummy/master wdir --noupdate -q
$ cd wdir
$ hg serve -n test -p $HGPORT -d --pid-file=hg.pid -E errors.log
$ cat hg.pid >> $DAEMON_PIDS
$ get-with-headers.py localhost:$HGPORT 'file/tip/x' | head -n 10
200 Script output follows
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN" "http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en-US">
<head>
<link rel="icon" href="/static/hgicon.png" type="image/png" />
<meta name="robots" content="index, nofollow" />
<link rel="stylesheet" href="/static/style-paper.css" type="text/css" />
<script type="text/javascript" src="/static/mercurial.js"></script>