Fix AttributeError when searching a remote by name

Running code like `'origin' in git.Repo('path/to/existing/repository').remotes`
raises an AttributeError instead of returning a boolean. This commit fixes that
behaviour by catching the error when doing an identity match on `IterableList`.

Author: cesarizu

Diff for: AUTHORS

@@ -32,5 +32,6 @@ Contributors are:
 -A. Jesse Jiryu Davis <jesse _at_ emptysquare.net>
 -Steven Whitman <ninloot _at_ gmail.com>
 -Stefan Stancu <stefan.stancu _at_ gmail.com>
+-César Izurieta <cesar _at_ caih.org>
 
 Portions derived from other open source works and are clearly marked.

Diff for: git/util.py

@@ -864,9 +864,12 @@ def __init__(self, id_attr, prefix=''):
 
     def __contains__(self, attr):
         # first try identity match for performance
-        rval = list.__contains__(self, attr)
-        if rval:
-            return rval
+        try:
+            rval = list.__contains__(self, attr)
+            if rval:
+                return rval
+        except (AttributeError, TypeError):
+            pass
         # END handle match
 
         # otherwise make a full name search