This should speed up pre-commit a bit as only files that are staged will
be processed, and clang-format and shellcheck are only invoked once, not
for every file. When no arguments are given (e.g. on CI), it still uses
'git ls-files'.
This needs '-o' to work correctly. Also update the shebang to bash in
some scripts as shellcheck was complaining about pipefail not being a
POSIX shell thing otherwise.
lint-shell-scripts searches over the repository looking for shell
scripts. On those found, shellcheck is run against them. If any linting
fails print those warnings and exit with a non-zero exit code.
Run this script automatically in Travis.
