knotserver: git: do not pass HEAD as a default arg to rev-list

when calling rev-list, HEAD was passed as a default argument:

git rev-list HEAD --count

but HEAD is only populated when we perform git.Open; it is zeroed out
when we perform git.PlainOpen:

git rev-list 00000... --count

this causes rev-list to exit with an error, when used in repos that are
git.PlainOpen'd.

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

knotserver/git/git.go

···
 	commits := []*object.Commit{}
 
 	output, err := g.revList(
+		g.h.String(),
 		fmt.Sprintf("--skip=%d", offset),
 		fmt.Sprintf("--max-count=%d", limit),
 	)
···
 
 func (g *GitRepo) TotalCommits() (int, error) {
 	output, err := g.revList(
+		g.h.String(),
 		fmt.Sprintf("--count"),
 	)
 	if err != nil {
···
 func (g *GitRepo) revList(extraArgs ...string) ([]byte, error) {
 	var args []string
 	args = append(args, "rev-list")
-	args = append(args, g.h.String())
 	args = append(args, extraArgs...)
 
 	cmd := exec.Command("git", args...)
 	cmd.Dir = g.path
 
-	return cmd.Output()
+	out, err := cmd.Output()
+	if err != nil {
+		if exitErr, ok := err.(*exec.ExitError); ok {
+			return nil, fmt.Errorf("%w, stderr: %s", err, string(exitErr.Stderr))
+		}
+		return nil, err
+	}
+
+	return out, nil
 }
 
 func (g *GitRepo) Commit(h plumbing.Hash) (*object.Commit, error) {