add script for git bisect on a performance test

.ci/atime/bisect-7687.sh

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+## git bisect old=equal perf, new=divergent.
+cp tests.R ~/tests.R
+git bisect start
+git bisect old 90f1c1e7b4811ceaef4848a2805a2295d9e5c5f2
+git bisect new master
+git bisect run bash ~/R/data.table/.ci/atime/run-7687.sh

.ci/atime/run-7687-ratios.R

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+library(data.table)
+tres <- readRDS("run-7687.rds")
+tref <- atime::references_best(tres)
+plot(tref)
+dcast(
+  tref$meas, N + unit ~ expr.name, value.var="empirical"
+)[, ratio := HEAD/Before][, inverse := 1/ratio][!is.na(ratio)][N==max(N)]
+tpred <- predict(tref, seconds=1, kilobytes=1e4)
+tpred <- predict(tref, seconds=1, kilobytes=2e4)
+plot(tpred)
+dcast(
+  tpred$prediction, unit ~ expr.name, value.var="N"
+)[, ratio := Before/HEAD][, inverse := 1/ratio][]

.ci/atime/run-7687.R

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+tinfo <- atime::atime_pkg_test_info("~/R/data.table")
+names(tinfo$test.call)
+tname <- "DT[by] fixed in #4558"
+tcall <- tinfo$test.call[[tname]]
+hist.sha.list <- as.list(tcall[c("Before", "Regression", "Fixed")])
+pr.sha.list <- tcall$sha.vec
+HEAD <- grep("HEAD", names(pr.sha.list), value=TRUE)
+tcall[c("Before", "Regression", "Fixed")] <- NULL
+tcall$sha.vec <- c(hist.sha.list, pr.sha.list)[c(HEAD, "Before")]
+names(tcall$sha.vec) <- c("HEAD","Before")
+##tcall$seconds.limit <- 1
+tres <- eval(tcall)
+saveRDS(tres, "run-7687.rds")
+library(data.table)
+print(meas_wide <- dcast(
+  tres$meas, N ~ expr.name, value.var="kilobytes"
+)[, ratio := HEAD/Before][max(which(!is.na(ratio)))])
+q(status=ifelse(meas_wide$ratio>1.4, 1, 0))

.ci/atime/run-7687.sh

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+ATIME=~/R/data.table/.ci/atime
+mkdir -p $ATIME
+cp ~/tests.R $ATIME/tests.R
+Rscript $ATIME/run-7687.R
+