If you follow these build instructions 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.