knotserver/git: add GitRepo.Refresh

after running a command externally that updates the git index, we need
to refresh go-git's repr of the git repo. remember to run this after
running something like `git am`.

Signed-off-by: oppiliappan <[e-mail]>

knotserver/git/git.go

···
 	return &g, nil
 }
 
+// re-open a repository and update references
+func (g *GitRepo) Refresh() error {
+	refreshed, err := PlainOpen(g.path)
+	if err != nil {
+		return err
+	}
+
+	*g = *refreshed
+	return nil
+}
+
 func (g *GitRepo) Commits(offset, limit int) ([]*object.Commit, error) {
 	commits := []*object.Commit{}