graft: add test showing --continue not preserving --log passed earlier
This patch shows that when we do `hg graft --continue`, the value of --log flag
passed when initial graft was run was not used.
Next patch will fix this and tests will help us in realising that.
Differential Revision: https://phab.mercurial-scm.org/D3661
#require docutils gettext
$ $TESTDIR/check-gendoc fr
checking for parse errors