fetch: fix recursive dependency fetching with -branch

The branch indication was being carried on to the recursive dependencies
because the global branch variable was getting shadowed before being
cleared.

Fixes #25

fetch.go

diff --git a/fetch.go b/fetch.go
index 13d90fb..5797bef 100644
--- a/fetch.go
+++ b/fetch.go
@@ -105,7 +105,7 @@
 		return err
 	}
 
-	branch, err := wc.Branch()
+	b, err := wc.Branch()
 	if err != nil {
 		return err
 	}
@@ -114,7 +114,7 @@
 		Importpath: path,
 		Repository: repo.URL(),
 		Revision:   rev,
-		Branch:     branch,
+		Branch:     b,
 		Path:       extra,
 		NoTests:    !tests,
 	}