diff -r 1764527af92e -r 41ac707322ba mercurial/scmutil.py
--- a/mercurial/scmutil.py	Fri Apr 13 09:19:38 2018 -0700
+++ b/mercurial/scmutil.py	Fri Apr 13 09:48:22 2018 -0700
@@ -444,6 +444,11 @@
     return node
 
 def isrevsymbol(repo, symbol):
+    """Checks if a symbol exists in the repo.
+
+    See revsymbol() for details. Raises error.LookupError if the symbol is an
+    ambiguous nodeid prefix.
+    """
     try:
         revsymbol(repo, symbol)
         return True