fix #H7103 - shop pricing

Player came across a stack of 2 gray stones in a shop and kicked one.
That one ended up with a different (in his case, lower) price once it
was separate.  This behavior only applies to non-glass gems which add
a price variation derived from internal ID (obj->o_id) number.  Make
splitting stacks always yield the same price per item in the new stack
as was being charged in the old stack by choosing a similar o_id.  Do
it for all splits (that can vary price by ID, so just non-glass gems),
not just ones performed inside shops.

He picked up the lower priced one and dropped it back on the original
higher priced one; the combined stack took on the lower price.  That
will no longer happen if they come from splitting a stack, but this
fix doesn't address merging with different prices when they start out
as separate stacks.  (Unpaid items won't merge in inventory if prices
are different, but shop-owned items will merge on floor.)

doc/fixes36.2

@@ -312,6 +312,8 @@ looking into an applied container with ':' showed prices of unpaid items but
 prevent achievement luckstone from merging with other luckstones if kicked or
 	dropped by monster (not applicable for dropped by player; achievement
 	would have been recorded and luckstone reverted to normal if picked up)
+when splitting a stack of gems where internal ID is used to make adjustment to
+	shop prices, pick an ID for new stack that yields same prices as old
 
 
 Fixes to Post-3.6.1 Problems that Were Exposed Via git Repository