knotserver/git: construct a more unique cache key

The previous cache key was just the path, so files that are often common
across git repos like '.gitignore' etc. show a cached "last commit" time
from another repository. Pretty funny.

This uses the current plumbing.Hash.String():path as the key which
should theoretically be unique to each repository.

knotserver/git/git.go

···
 }
 
 func (g *GitRepo) LastCommitForPath(path string) (*object.Commit, error) {
+	cacheKey := fmt.Sprintf("%s:%s", g.h.String(), path)
 	cacheMu.RLock()
-	if commit, found := commitCache.Get(path); found {
+	if commit, found := commitCache.Get(cacheKey); found {
 		cacheMu.RUnlock()
 		return commit.(*object.Commit), nil
 	}
···
 	}
 
 	cacheMu.Lock()
-	commitCache.Set(path, commit, 1)
+	commitCache.Set(cacheKey, commit, 1)
 	cacheMu.Unlock()
 
 	return commit, nil