context: clarify that only one attempt is made to interpret changeid

We can now tell what type of revision specifier we have just by
looking at it (we no longer attempt to interpret it in one way after
the other -- that's now in scmutil.revsymbol()). Let's clarify this in
the code by swithing to if/elif.

Differential Revision: https://phab.mercurial-scm.org/D3451

mercurial/context.py

@@ -389,22 +389,22 b' class changectx(basectx):'
                 self._node = repo.changelog.node(changeid)
                 self._rev = changeid
                 return
-            if changeid == 'null':
+            elif changeid == 'null':
                 self._node = nullid
                 self._rev = nullrev
                 return
-            if changeid == 'tip':
+            elif changeid == 'tip':
                 self._node = repo.changelog.tip()
                 self._rev = repo.changelog.rev(self._node)
                 return
-            if (changeid == '.'
-                or repo.local() and changeid == repo.dirstate.p1()):
+            elif (changeid == '.'
+                  or repo.local() and changeid == repo.dirstate.p1()):
                 # this is a hack to delay/avoid loading obsmarkers
                 # when we know that '.' won't be hidden
                 self._node = repo.dirstate.p1()
                 self._rev = repo.unfiltered().changelog.rev(self._node)
                 return
-            if len(changeid) == 20:
+            elif len(changeid) == 20:
                 try:
                     self._node = changeid
                     self._rev = repo.changelog.rev(changeid)
@@ -421,7 +421,7 b' class changectx(basectx):'
                         msg = _("working directory has unknown parent '%s'!")
                         raise error.Abort(msg % short(changeid))
 
-            if len(changeid) == 40:
+            elif len(changeid) == 40:
                 try:
                     self._node = bin(changeid)
                     self._rev = repo.changelog.rev(self._node)