Running 'make test' dies due to no .git when building from a .tar.gz

If you [follow these build instructions](https://github.com/github/git-sizer/blob/master/docs/BUILDING.md#build-using-make) and run `make test` on one of the `.tar.gz` releases you get:

```
$ make test; echo $?
[...]
Messages: command failed; output: "error: couldn't open Git repository: git rev-parse failed: fatal: not a git repository (or any of the parent directories): .git\n\n"
[...]
make: *** [gotest] Error 1
2
```

Just running `git init` in the build directory works around it, but I don't know how much this breaks the tests, if at all:

```
$ make test; echo $?
[...]
ok github.com/github/git-sizer 0.123s
ok github.com/github/git-sizer/counts (cached)
? github.com/github/git-sizer/git [no test files]
? github.com/github/git-sizer/isatty [no test files]
? github.com/github/git-sizer/meter [no test files]
? github.com/github/git-sizer/sizes [no test files]
0
```

So now I do `git init` in my packaging script, but it would be neat if this worked out of the box.