NewRepository: don't join paths if gitDir is already absolute

If the output of `git -C path rev-parse --git-dir` is already an
absolute path (e.g., if `git-sizer` is run from within a subdirectory
of a working copy), then it is incorrect to call `filepath.Join()`
because that function doesn't treat absolute paths specially. So check
`filepath.IsAbs()` first, and only call `filepath.Join()` if the path
is not already absolute.

Fixes #10.

Author: mhagger

git/git.go

@@ -90,7 +90,10 @@ func NewRepository(path string) (*Repository, error) {
 			return nil, err
 		}
 	}
-	gitDir := filepath.Join(path, string(bytes.TrimSpace(out)))
+	gitDir := string(bytes.TrimSpace(out))
+	if !filepath.IsAbs(gitDir) {
+		gitDir = filepath.Join(path, gitDir)
+	}
 	repo := &Repository{
 		path: gitDir,
 	}