range-set invariants are: ranges must be (1) non-empty, (2) disjoint,
(3) sorted in ascending order.

line_log_data_insert() breaks the non-empty invariant under the
following conditions: the incoming range is empty and the pathname
attached to the range has not yet been encountered. In this case,
line_log_data_insert() assigns the empty range to a new line_log_data
record without taking any action to ensure that the empty range is
eventually folded out.  Subsequent range-set functions crash or throw an
assertion failure upon encountering such an anomaly.  Fix this bug.
Signed-off-by: NEric Sunshine <sunshine@sunshineco.com>
Acked-by: NThomas Rast <trast@inf.ethz.ch>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>