knotserver: git: fetch last commit time against current plumbing.Hash

This fixes the bug where paths wouldn't show up in non-default branches.

We were running 'git log' against the bare repository that was pointing
to the default branch, which would then return no output for that
specific path since -- understandably -- the path doesn't exist on the
default branch. This change simply passes the current g.h
(plumbing.Hash) to 'git log' which resolves the path in that ref.

knotserver/git/git.go

···
 	}
 	cacheMu.RUnlock()
 
-	cmd := exec.Command("git", "-C", g.path, "log", "-1", "--format=%H %ct", "--", path)
+	cmd := exec.Command("git", "-C", g.path, "log", g.h.String(), "-1", "--format=%H %ct", "--", path)
 
 	var out bytes.Buffer
 	cmd.Stdout = &out

knotserver/git/tree.go

···
 
 import (
 	"fmt"
+	"time"
 
 	"github.com/go-git/go-git/v5/plumbing/object"
 	"tangled.sh/tangled.sh/core/types"
···
 		lastCommit, err := g.LastCommitForPath(fpath)
 		if err != nil {
 			fmt.Println("error getting last commit time:", err)
-			continue
+			// We don't want to skip the file, so worst case lets just
+			// populate it with "defaults".
+			lastCommit = &types.LastCommitInfo{
+				Hash:    g.h,
+				Message: "",
+				When:    time.Now(),
+			}
 		}
 
 		nts = append(nts, types.NiceTree{