hg: make _local() behave consistently on Python 3.8 (issue6287)

Python 3.8 makes os.path.isfile quietly eat "path invalid" errors and
return False instead of allowing the exception to propagate. Given
that this is a change from 2018 (sigh) and it's mentioned in the
release notes (double sigh) we're definitely too late to complain to
Python about the behavior change, so open-code part of
os.path.isfile() in this method so we can catch invalid-path errors
and handle them appropriately. I confirmed that posixpath and ntpath
both delegate to genericpath, which uses os.stat() under the covers.

Differential Revision: https://phab.mercurial-scm.org/D8302

mercurial/hg.py

@@ -60,12 +60,19 b' def _local(path):'
     path = util.expandpath(util.urllocalpath(path))
 
     try:
-        isfile = os.path.isfile(path)
+        # we use os.stat() directly here instead of os.path.isfile()
+        # because the latter started returning `False` on invalid path
+        # exceptions starting in 3.8 and we care about handling
+        # invalid paths specially here.
+        st = os.stat(path)
+        isfile = stat.S_ISREG(st.st_mode)
     # Python 2 raises TypeError, Python 3 ValueError.
     except (TypeError, ValueError) as e:
         raise error.Abort(
             _(b'invalid path %s: %s') % (path, pycompat.bytestr(e))
         )
+    except OSError:
+        isfile = False
 
     return isfile and bundlerepo or localrepo