subrepo: handle missing subrepo spec file as removed
Otherwise, all commands involving a dirstate walk will abort when trying to
readone of them. Deleting .hgsub basically breaks a repository.
subrepo: use subprocess directly to avoid python 2.6 bug
Using svn subrepos on MacOSX with native python 2.6.1 results in a lot of
unexpected output caused by:
http://bugs.python.org/
issue5099
subprocess.Popen.__del__ causes AttributeError (os module == None)
Avoiding dangling Popen instance solves the issue.