safehasattr: pass attribute name as string instead of bytes

This is a step toward replacing `util.safehasattr` usage with plain `hasattr`.
The builtin function behave poorly in Python2 but this was fixed in Python3.

These change are done one by one as they tend to have a small odd to trigger
puzzling breackage.

mercurial/hg.py

@@ -317,7 +317,7 b' def sharedreposource(repo):'
     if repo.sharedpath == repo.path:
         return None
 
-    if util.safehasattr(repo, b'srcrepo') and repo.srcrepo:
+    if util.safehasattr(repo, 'srcrepo') and repo.srcrepo:
         return repo.srcrepo
 
     # the sharedpath always ends in the .hg; we want the path to the repo