hg-to-git: fix parent analysis

hg-to-git: fix parent analysis

Fix a bug in the hg-to-git convertor introduced by commit
1bc7c13af9f936aa80893100120b542338a10bf4: when searching the changeset
parents, 'hg log' returns an extra space at the end of the line, which
confuses the .split(' ') based tokenizer:

    Traceback (most recent call last):
      File "hg-to-git.py", line 123, in <module>
          hgchildren[mparent] += ( str(cset), )
      KeyError: ''

Signed-off-by: Stelian Pop <stelian@popies.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>