gdb/python: add gdb.history_count function

gdb/python: add gdb.history_count function

Add a new function gdb.history_count to the Python api, this function
returns an integer, the number of items in GDB's value history.

This is useful if you want to pull items from the history by their
absolute number, for example, if you wanted to show a complete history
list.  Previously we could figure out how many items are in the
history list by trying to fetch the items, and then catching the
exception when the item is not available, but having this function
seems nicer.