githelp: use revsymbol() for looking up symbol

I don't know if we should be using revsingle() here, so I was
conservative and switched to revsymbol().

Differential Revision: https://phab.mercurial-scm.org/D3156

hgext/githelp.py

@@ -26,6 +26,7 b' from mercurial import ('
     error,
     fancyopts,
     registrar,
+    scmutil,
 )
 from mercurial.utils import (
     procutil,
@@ -253,7 +254,7 b' def ispath(repo, string):'
     too many ways to spell revisions in git for us to reasonably catch all of
     them, so let's be conservative.
     """
-    if string in repo:
+    if scmutil.isrevsymbol(repo, string):
         # if it's definitely a revision let's not even check if a file of the
         # same name exists.
         return False