Print an extra message indicating which commit caused the slowdown

<naxieAlDle> bench: 2017.02,HEAD for ^100_000 { $ = 'st' ~~ m:i/st/ }; say now - INIT now
<benchable6> naxieAlDle, starting to benchmark the 2 given commits
<naxieAlDle> let's see if it works
<benchable6> naxieAlDle, benchmarked the given commits, now zooming in on performance differences
<benchable6> naxieAlDle, https://gist.github.com/8b95​a45f7697669881ed75f030f05145
<naxieAlDle> that's better :)
<naxieAlDle> I wonder if it can actually find it
<naxieAlDle> bench: 61a65ce,26e6993 for ^100_000 { $ = 'st' ~~ m:i/st/ }; say now - INIT now
<benchable6> naxieAlDle, starting to benchmark the 2 given commits
<benchable6> naxieAlDle, benchmarked the given commits, now zooming in on performance differences
<benchable6> naxieAlDle, ¦61a65ce: «2.0492» ¦26e6993: «2.7941»
<naxieAlDle> ah!! It already did!!

Otherwise it is unclear whether it stopped because of the time limit or if it actually found the right commit.

Start by tracing the benchmark command shown in the issue and the code that emits its progress messages. Make the output explicitly say which commit caused the slowdown, and verify that it distinguishes a found commit from stopping at the time limit using the benchmark example.