bdiff: handle the possibility of an integer overflow when allocating Differential Revision: https://phab.mercurial-scm.org/D1877

svnsubrepo: check if subrepo is missing when checking dirty state (issue5657) During an hg convert command, subrepos are not checked out. However, a manifestmerge may occur during an hg convert if there is a merge commit, which in particular will check the dirty state of subrepos. Before this change, the lack of a checked out subrepo would error out. This check for svn subrepos is modelled after 2fdea636f254 for git subrepos.