On some systems, /bin/sh is a bash symlink and running it will launch
bash in sh compatibility mode. However, bash-specific syntax will still
work in this mode (for example using == or pipefail).

However, on systems where /bin/sh is a symlink to another shell (think:
Debian-based) they might not have those bashisms.

Set the shell in the Makefile, so that it uses bash (since it is already
depending on bash, even though it is calling it as /bin/sh by default),
and add a shebang to pre-commit.sh for the same reason.

Signed-off-by: Rick Elrod <rick@elrod.me>