contrib/completion: install Bash completion

contrib/completion: install Bash completion

The shell completion scripts in "contrib/completion" are being tested,
but none of our build systems support installing them. This is somewhat
confusing for Meson, where users can explicitly enable building these
scripts via `-Dcontrib=completion`. This option only controlls whether
the completions are built and tested against, where "building" is a bit
of an euphemism for "copying them into the build directory".

Teach both our Makefile and Meson to install our Bash completion script.
For now, this is the only completion script that we're installing given
that Bash completions "just work" with a canonical well-known location
nowadays. Other completion scripts, like for example the one for zsh,
don't have a well-known location and/or require extra steps by the user
to make them available. As such, we skip installing these scripts for
now, but we may do so in the future if we ever figure out a proper way
to do this.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>