use branch revision during worktree add (v4)

This is a port of the v3 commit 2c3bb035f6

cmd/git-sync/main.go

@@ -896,11 +896,6 @@ func (git *repoSync) AddWorktreeAndSwap(ctx context.Context, hash string) error
 		return nil
 	}
 
-	// GC clone
-	if _, err := git.run.Run(ctx, git.root, git.cmd, "gc", "--prune=all"); err != nil {
-		return err
-	}
-
 	// Make a worktree for this exact git hash.
 	worktreePath := filepath.Join(git.root, hash)
 
@@ -912,12 +907,17 @@ func (git *repoSync) AddWorktreeAndSwap(ctx context.Context, hash string) error
 		return err
 	}
 
-	_, err := git.run.Run(ctx, git.root, git.cmd, "worktree", "add", worktreePath, "origin/"+git.branch, "--no-checkout")
+	_, err := git.run.Run(ctx, git.root, git.cmd, "worktree", "add", "--detach", worktreePath, hash, "--no-checkout")
 	git.log.V(0).Info("adding worktree", "path", worktreePath, "branch", fmt.Sprintf("origin/%s", git.branch))
 	if err != nil {
 		return err
 	}
 
+	// GC clone
+	if _, err := git.run.Run(ctx, git.root, git.cmd, "gc", "--prune=all"); err != nil {
+		return err
+	}
+
 	// The .git file in the worktree directory holds a reference to
 	// /git/.git/worktrees/<worktree-dir-name>. Replace it with a reference
 	// using relative paths, so that other containers can use a different volume

test_e2e.sh

@@ -553,6 +553,42 @@ assert_file_eq "$ROOT"/link/file "$TESTCASE 1"
 # Wrap up
 pass
 
+##############################################
+# Test switching branch after depth=1 checkout
+##############################################
+testcase "branch-switch"
+# First sync
+echo "$TESTCASE" > "$REPO"/file
+git -C "$REPO" commit -qam "$TESTCASE"
+GIT_SYNC \
+    --one-time \
+    --repo="file://$REPO" \
+    --branch=e2e-branch \
+    --depth=1 \
+    --root="$ROOT" \
+    --link="link" \
+    > "$DIR"/log."$TESTCASE" 2>&1
+assert_link_exists "$ROOT"/link
+assert_file_exists "$ROOT"/link/file
+assert_file_eq "$ROOT"/link/file "$TESTCASE"
+BRANCH="${TESTCASE}2"
+git -C "$REPO" checkout -q -b $BRANCH
+echo "$TESTCASE 2" > "$REPO"/file
+git -C "$REPO" commit -qam "$TESTCASE 2"
+GIT_SYNC \
+    --one-time \
+    --repo="file://$REPO" \
+    --branch=$BRANCH \
+    --rev=HEAD \
+    --root="$ROOT" \
+    --link="link" \
+    >> "$DIR"/log."$TESTCASE" 2>&1
+assert_link_exists "$ROOT"/link
+assert_file_exists "$ROOT"/link/file
+assert_file_eq "$ROOT"/link/file "$TESTCASE 2"
+# Wrap up
+pass
+
 ##############################################
 # Test tag syncing
 ##############################################