Check for non-gccrs things in the commits we cherry-pick

[CohenArthur]

from @dkm:

which is the same as doing git rev-list --no-merges --reverse ^gcc/trunk -- gcc/rust gcc/testsuite/rust libgrust
Hmm, if you have a commit that touches rust/foo.c and gcc/tree.h, I think it will show up in your list, but I may be wrong

we need to check the content of each commit we cherry-pick. if content in the commit belongs to something other than the Rust frontend bits, it should be flagged and reported when opening the PR