Evaluate symlinks when running FetchAll.

For SlothFS, git repositories are cold and bulky data, so they are
good candidate to put onto a spinning disk via symlinks. To ensure
that we still run FetchAll over these, deref the gitCache directory
before the directory walk.

Tested:
  Manually verified that git fetches are being executed again.

Change-Id: I6965e6a525485cec10678be46f640fafaf57236d
diff --git a/cache/gitcache.go b/cache/gitcache.go
index fa8c827..8625e4f 100644
--- a/cache/gitcache.go
+++ b/cache/gitcache.go
@@ -88,8 +88,13 @@
 
 // FetchAll finds all known repos and runs git-fetch on them.
 func (c *gitCache) FetchAll() error {
+	dir, err := filepath.EvalSymlinks(c.dir)
+	if err != nil {
+		return err
+	}
+
 	var dirs []string
-	if err := filepath.Walk(c.dir, func(n string, fi os.FileInfo, err error) error {
+	if err := filepath.Walk(dir, func(n string, fi os.FileInfo, err error) error {
 		if fi.IsDir() && strings.HasSuffix(n, ".git") {
 			dirs = append(dirs, n)
 			return filepath.SkipDir