Test goes fail when git config is not set

[jmkim]

The test goes fail with git exit status 128 while committing when git config is not set globally or locally.

Here is an error log with go test -test.v -check.v -gocheck.v:

----------------------------------------------------------------------
FAIL: worktree_test.go:232: RepositorySuite.TestPullAdd

worktree_test.go:248:
    ExecuteOnPath(c, path,
        "touch foo",
        "git add foo",
        "git commit -m foo foo",
    )
repository_test.go:2540:
    c.Assert(err, IsNil)
... value *exec.ExitError = &exec.ExitError{ProcessState:(*os.ProcessState)(0xc00268de40), Stderr:[]uint8(nil)} ("exit status 128")

OOPS: 253 passed, 1 FAILED

I'll make a patch very soon and will send PR here.