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
#require docutils gettext
$ $TESTDIR/check-gendoc zh_TW
checking for parse errors