- move resolution of "starting rev" for --sql mode into

- move resolution of "starting rev" for --sql mode into
get_current_heads() directly; therefore we don't need to
do this in alembic.command, which we were doing for stamp but
not downgrade/upgrade.  The slight change here is that the
context.get_starting_revision_argument() method will
return an abbreviated starting rev as abbreviated in
all cases, including the stamp command, where we previously
were converting a stamp argument first, but not for the
upgrade or downgrade commands.
- Fixed bug where using a partial revision identifier as the
"starting revision" in ``--sql`` mode in a downgrade operation
would fail to resolve properly.  fixes #269