feat: allow hash table with file list for spack cherry-picks

We try to stick to a spack release with selected cherry-picks. Occasionally, spack has a treewide commit that breaks this because all subsequent cherry-picks need the treewide commit to apply successfully, and applying the treewide commit as a cherry-pick would require essentially every commit since the release. This PR adds the ability to apply partial cherry-picks, restricted to just the specified files. This allows is to apply the treewide commit on only the files that need it.

spack.sh

@@ -29,11 +29,12 @@ c07ddf83c32b7129247fe90eed486dd844047087
 1255620a14afa3ad4aad681a847a3a1704141976
 0fed2d66bf0eec799707dd1b88ac9419f6ae14e1
 963e2ca82883cdc1287f1035c15d1a7e9a6fe612
+19c20563cc86140aaf352d72079bd9de292be0ac
 ---
 ## Optional hash table with comma-separated file list
 read -r -d '' SPACK_CHERRYPICKS_FILES <<- \
 --- || true
-[19c20563cc86140aaf352d72079bd9de292be0ac]=var/spack/repos/builtin/packages/pythia8/package.py
+[19c20563cc86140aaf352d72079bd9de292be0ac]=var/spack/repos/builtin/packages/hepmc3/package.py,var/spack/repos/builtin/packages/pythia8/package.py
 ---
 ## Ref: https://github.com/spack/spack/commit/[hash]
 ## [hash]: [description]