git: consistently use str for parents when rebuilding the index database

The tests show no changes, but when these values are overwritten shortly after
when a git commit object is available, that uses str.  It seems better to use
that for consistency.

It does materially affect the database though, because the old value stored was
`X'3030..3030'` and is now '00..00' when the changelog table is dumped in
sqlite3.  There is one query that specifies the parents, but it passes the non
null hashes as str, so it worked as expected.  That likely explains the lack of
test changes.

Differential Revision: https://phab.mercurial-scm.org/D10527

hgext/git/index.py

@@ -6,7 +6,6 b' import sqlite3'
 
 from mercurial.i18n import _
 from mercurial.node import (
-    nullhex,
     nullid,
 )
 
@@ -281,7 +280,7 b' def _index_repo('
     for pos, commit in enumerate(walker):
         if prog is not None:
             prog.update(pos)
-        p1 = p2 = nullhex
+        p1 = p2 = gitutil.nullgit
         if len(commit.parents) > 2:
             raise error.ProgrammingError(
                 (