subrepo: handle unexpected file types from git gracefully

This was flagged by pytype because `tar.extractfile(...)` can return None if the
entry is not a file or symlink.  I don't think that git supports other types,
but better safe than sorry.

Differential Revision: https://phab.mercurial-scm.org/D10179

mercurial/subrepo.py

@@ -1876,7 +1876,12 b' class gitsubrepo(abstractsubrepo):'
             if info.issym():
                 data = info.linkname
             else:
-                data = tar.extractfile(info).read()
+                f = tar.extractfile(info)
+                if f:
+                    data = f.read()
+                else:
+                    self.ui.warn(_(b'skipping "%s" (unknown type)') % bname)
+                    continue
             archiver.addfile(prefix + bname, info.mode, info.issym(), data)
             total += 1
             progress.increment()