Fix slice logic for less-then-frame tsdb overlap

When the tsdb has a last datum that is in the past less then a "frame's
worth" of sample steps we need to slice out only the data from the
latest frame that doesn't overlap; this fixes that slice logic..
Previously i dunno wth it was doing..

piker/data/feed.py

@@ -208,27 +208,24 @@ def diff_history(
 
 ) -> np.ndarray:
 
+    to_push = array
+
     if last_tsdb_dt:
         s_diff = (start_dt - last_tsdb_dt).seconds
 
-        to_push = array[:s_diff]
-
         # if we detect a partial frame's worth of data
         # that is new, slice out only that history and
         # write to shm.
-        if abs(s_diff) < len(array):
+        if (
+            s_diff < 0
+            and abs(s_diff) < len(array)
+        ):
             log.info(
                 f'Pushing partial frame {to_push.size} to shm'
             )
-            # assert last_tsdb_dt > start_dt
-            # selected = array['time'] > last_tsdb_dt.timestamp()
-            # to_push = array[selected]
-            # return to_push
+            to_push = array[abs(s_diff):]
 
-        return to_push
-
-    else:
-        return array
+    return to_push
 
 
 async def start_backfill(
@@ -248,6 +245,17 @@ async def start_backfill(
         # back to what is recorded in the tsdb
         array, start_dt, end_dt = await hist(end_dt=None)
 
+        times = array['time']
+
+        # sample period step size in seconds
+        step_size_s = (
+            pendulum.from_timestamp(times[-1]) -
+            pendulum.from_timestamp(times[-2])
+        ).seconds
+
+        # "frame"'s worth of sample period steps in seconds
+        frame_size_s = len(array) * step_size_s
+
         to_push = diff_history(
             array,
             start_dt,
@@ -267,13 +275,6 @@ async def start_backfill(
         # let caller unblock and deliver latest history frame
         task_status.started((shm, start_dt, end_dt, bf_done))
 
-        times = array['time']
-        step_size_s = (
-            pendulum.from_timestamp(times[-1]) -
-            pendulum.from_timestamp(times[-2])
-        ).seconds
-        frame_size_s = len(to_push) * step_size_s
-
         if last_tsdb_dt is None:
             # maybe a better default (they don't seem to define epoch?!)