[flang] Add script to flatten git history for llvm monorepo submission (flang-compiler/f18#854)

This script, when run on a checkout of the f18 repository, takes the
current origin/master and makes a branch called "new" with a rewritten
history;

The "new" branch has a flat git history (that is, a series of commits
with only one parent). Flattening is done for merge commits by taking
the content of the commit as it is at the merge commit.

Original-commit: flang-compiler/f18@d9871fa9eb2304c4761a4a818187553396bb8924
Reviewed-on: https://github.com/flang-compiler/f18/pull/854