subrepo: add shortid() method to subrepo classes

This method takes an "id" (e.g. a revision id) and returns a "short" version
(e.g. a short revision id).

This will be used on the next revision to fix a small bug in the way that the
text on the promptchoice shown when a subrepo diverges is generated.

mercurial/subrepo.py

@@ -501,6 +501,9 b' class abstractsubrepo(object):'
             % (substate[0], substate[2]))
         return []
 
+    def shortid(self, revid):
+        return revid
+
 class hgsubrepo(abstractsubrepo):
     def __init__(self, ctx, path, state):
         self._path = path
@@ -866,6 +869,9 b' class hgsubrepo(abstractsubrepo):'
             pats = []
         cmdutil.revert(ui, self._repo, ctx, parents, *pats, **opts)
 
+    def shortid(self, revid):
+        return revid[:12]
+
 class svnsubrepo(abstractsubrepo):
     def __init__(self, ctx, path, state):
         self._path = path
@@ -1561,6 +1567,9 b' class gitsubrepo(abstractsubrepo):'
         deleted = unknown = ignored = clean = []
         return modified, added, removed, deleted, unknown, ignored, clean
 
+    def shortid(self, revid):
+        return revid[:7]
+
 types = {
     'hg': hgsubrepo,
     'svn': svnsubrepo,