parameterize pre-commit hook by branch instead of using the literal "master"

[cosmicexplorer]

See #7214 (comment). It might end up with cleaner and more readable code if we settle on using a variable like BASE_BRANCH instead of typing out master literally each time. e.g.:

# The branch to calculate changed targets and files against.
BASE_BRANCH="${BASE_BRANCH:-master}"
# Test if this git repo contains the specified base branch. When travis builds a tag, it does so in
# a shallow clone without master fetched, so we currently can't calculate changes against it.
if git rev-parse --verify "$BASE_BRANCH" &>/dev/null; then echo "* Checking imports" && ./build-support/bin/isort.sh || \
  # ...

[Eric-Arellano]

We use the value git_merge_base today.