Browse: ensure we erase all of old type name when updating position data.

Browse: ensure we erase all of old type name when updating position data.

Top-right of the merge window identifies the current location in
the merge.  It is variable length and right justified, so we need to make
sure we erase start of old value before writing new value.
Old code only allowed for a difference of three which didn't cope
with, for example, change from AlreadyApplied to Changed.
So be more thorough.

Signed-off-by: NeilBrown <neilb@suse.de>