Bench: allow overriding new and old coq_commit through env variables

dev/bench/bench.sh

@@ -72,12 +72,12 @@ check_variable () {
 : "${coq_opam_packages:=coq-bignums coq-hott coq-performance-tests-lite coq-engine-bench-lite coq-mathcomp-ssreflect coq-mathcomp-fingroup coq-mathcomp-algebra coq-mathcomp-solvable coq-mathcomp-field coq-mathcomp-character coq-mathcomp-odd-order coq-math-classes coq-corn coq-compcert coq-equations coq-metacoq-template coq-metacoq-pcuic coq-metacoq-safechecker coq-metacoq-erasure coq-metacoq-translations coq-color coq-coqprime coq-coqutil coq-bedrock2 coq-rewriter coq-fiat-core coq-fiat-parsers coq-fiat-crypto-with-bedrock coq-unimath coq-coquelicot coq-iris-examples coq-verdi coq-verdi-raft coq-fourcolor coq-rewriter-perf-SuperFast coq-perennial coq-vst coq-category-theory}"
 : "${coq_native:=}"
 
+: "${new_coq_commit:=$(git rev-parse HEAD^2)}"
+: "${old_coq_commit:=$(git merge-base HEAD^1 $new_coq_commit)}"
+
 new_ocaml_switch=ocaml-base-compiler.$new_ocaml_version
 old_ocaml_switch=ocaml-base-compiler.$old_ocaml_version
 
-new_coq_commit=$(git rev-parse HEAD^2)
-old_coq_commit=$(git merge-base HEAD^1 $new_coq_commit)
-
 if echo "$num_of_iterations" | grep '^[1-9][0-9]*$' 2> /dev/null > /dev/null; then
     :
 else