subrepo: add dirtyreason to centralize composing dirty reason message This patch newly adds "dirtyreason()" to centralize composing dirty reason message like "uncommitted changes in subrepository 'xxxx'". There are 3 similar messages below, and this patch is a part of preparations for unifying them into (1), too. 1. uncommitted changes in subrepository 'XXXX' 2. uncommitted changes in subrepository XXXX 3. uncommitted changes in subrepo XXXX This patch chooses adding new method "dirtyreason()" instead of making "dirty()" return "reason string", because: - some of existing "dirty()" implementation is too complicated to do so simply, and - ill-mannered 3rd party subrepo classes, of which "dirty()" doesn't return "reason string", cause meaningless message (even though it is rare case)