Include git hash in bat -V and bat --version output if present

[Enselic]

I had to use a lazy_static due to the strange clap API that only accepts a reference to a version string. And, in our current code, only a 'static reference to a version string is accepted. Code could of course be refactored, but it would be way more messy.

I have several binaries of bat lying around everywhere, and with this PR, I (and anyone else of course) can just do bat -V to find out exactly what version of bat is at hand. Right now I have to do bat --diagnostics | head, which is too unergonomic for my taste.

I could of course have a local script to avoid repetition, but who wants to local scripts for these kinds of things?

The output looks like this:

% cargo run -- -V
bat 0.18.3 (9291a79)

% cargo run -- --version
bat 0.18.3 (9291a79)

which is identical to the output format in --diagnostic:

% cargo run -- --diagnostic | head -n 3
#### Software version

bat 0.18.3 (9291a79)

If no git repo is available at build time, the output is the same as before:

% touch src/bin/bat/main.rs # force rebuild

% GIT_DIR=/not-present cargo run -- -V
bat 0.18.3

% GIT_DIR=/not-present cargo run -- --version
bat 0.18.3

[sharkdp]

Thanks!