Source archives should contain Git commit hash and date

[nodakai]

Can we include the output of git log -1 into the source archives? It can be important for beta and nightly.

Ideally the download page should also display them.

• https://www.rust-lang.org/downloads.html

[alexcrichton]

This should be a pretty easy bug to tackle for anyone aspiring to do so. Let's just create a file called VERSION in our tarballs with basically the output of rustc -v

[brson]

This will require editing the makefiles in mk/dist.mk as well as the rustbuild build system in src/rustbuild. The version info is in CFG_VERSION. I realized that the installers already include this info in a file called 'version', so we can just duplicate that logic. Use the same lower case naming for consistency.