When no '.pre-commit-config.yaml' file exists while `pre-commit` hooks
are enabled, `pre-commit` returns an error and the action is aborted.
This is a very common scenario when pre-commit is added later on a
project and the user wants to work on a previous branch where the
configuration file does not exist.
This commits allow the user to optionally install the `pre-commit` hooks
with an option to allow a missing configuration and trigger only the
legacy pre-commit hooks (if any) when it is missing.
When pushing a branch that does not exist on the remote repository,
instead of blindly running the checks on every file, this commit locates
the first ancestor not present on the remote repository and uses its
parent as the source of the fileset calculation. If it has no parents,
then the remote repository has no common commits and the checks should
be run on all files.
Anthony Sottile