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the combined_after_bytes.  Use DEC than INC when leaving
a marker before the insertion.
(count_combining_before): Return number of bytes from start
of insertion that will combine.
(insert_1_both): Compensate for change in count_combining_before.
Delete intervals for the combined_after_bytes.
Compensate for that when updating other intervals.
(insert_from_buffer_1, replace_range): Likewise.
(adjust_after_replace): Likewise.
(insert_from_string_1): Likewise.  Also handle intervals
for inserting just part of string.