feature idea: detect git short version hash during build and embed into binary

Nobody has claimed this yet.

In my apps I usually do something like:

```
var Version string

func main() {
os.Exit(httpserver.Run(Version))
}
```

Then at build time (in my Makefile) I do this:

```
VERSION := $(shell git describe --always --dirty=-dirty)
GOLDFLAGS += -X main.Version=$(VERSION)
GOFLAGS := -ldflags "$(GOLDFLAGS)"
```

This embeds the version string into the app so that I can access it, which is useful for me because then in production I can access this value and put it in my footers and error messages so that I know precisely which commit is running.

It would be cool if `bud build` could detect if it's building out of a git repo, and embed the git shorthash into the built binary in this way. It seems that you could skip the `ldflags` part and simply "hardcode" the git shorthash into the main.go via the main.gotext template at build filesystem generation time.

Anyway, just an idea, and I'm happy to take a crack at making a patch for this when I have some time, I just wanted to write it down while it's in my head.

Start with the `bud build` flow, the Makefile build settings, and the `main.go` and `main.gotext` locations mentioned in the issue. Trace how the build filesystem is generated and determine how a Git short hash could be embedded when building from a repository. Done means the built binary exposes the commit hash while builds outside Git remain supported.

Written by the indexing model from the issue text.