BUG: Fix an issue where history fails during forward-filling on the first
minute when the only supplied HistorySpec is a 1m price history.

The bug occurred because there is a special case in the initial window setup
code for handling the case where only a length-1 history is used for a given
frequency.  Previously, the code was incorrectly calculating the period end
using a hard-coded expression for the end of the day (the correct behavior for
a length-1 '1d' history), and then using the frequency object to calculate the
period start for the window.  In the case of length 1 '1m' data, this resulted
in an initial window whose start and end was the last minute of the day rather
than the first minute of the day.  For non-price fields, this error doesn't
matter, because the window is only used for rolling digests (which doesn't
happen when there's only a length-1 history), and for the forward-filling logic
(which only happens on price fields).  For a length-1 '1m' price, however, the
incorrect window causes us to attempt to forward-fill an empty panel, resulting
in an IndexError when we do an iloc[0] on a length-0 axis.