Fix cumulative percentage calculation for recursive functions in the new
sampling profiler. When profiling recursive functions, cumulative statistics
(cumul%, cumtime) could exceed 100% because each recursive frame in a stack
was counted separately. For example, a function recursing 500 times in every
sample would show 50000% cumulative presence. The fix deduplicates locations
within each sample so cumulative stats correctly represent "percentage of
samples where this function was on the stack". Patch by Pablo Galindo.
