fix 'convert' with single commit repositories
The attached patch passes the three 'convert' testcases, and also is
able to successfully convert the git.git repository.
import os
from mercurial import hg, ui, commands
u = ui.ui()
repo = hg.repository(u, 'test1', create=1)
os.chdir('test1')
repo = hg.repository(u, '.') # FIXME: can't lock repo without doing this
# create 'foo' with fixed time stamp
f = file('foo', 'w')
f.write('foo\n')
f.close()
os.utime('foo', (1000, 1000))
# add+commit 'foo'
repo.add(['foo'])
repo.commit(text='commit1', date="0 0")
print "workingfilectx.date =", repo.workingctx().filectx('foo').date()