Don't use Mercurial if 'hg root' fails.
Don't use Mercurial if 'hg root' fails.

We were assuming the current directory was a Mercurial checkout even when
'hg root' failed. I can't find a good reason we're doing that (perhaps
some old compatibility issue?) but it prevents falling back on other
SCMTools if there's any valid local Mercurial user configuration.

clach04/rbtools,davidt/rbtools,reviewboard/rbtools,datjwu/rbtools,datjwu/rbtools,Khan/rbtools,halvorlu/rbtools,halvorlu/rbtools,davidt/rbtools,haosdent/rbtools,1tush/rbtools,Khan/rbtools,beol/rbtools,clach04/rbtools_0.4.1,davidt/rbtools,reviewboard/rbtools,haosdent/rbtools,beol/rbtools,halvorlu/rbtools,reviewboard/rbtools,datjwu/rbtools,haosdent/rbtools,bcelary/rbtools,beol/rbtools,clach04/old_rbtools_0.3.2