Merge pull request 'agentws: check out --branch from origin when it exists' (#17) from benvin/agentws-branch-checkout into main

Reviewed-on: #17

internal/agent/git.go

@@ -109,6 +109,37 @@ func GitRemoteBranchExists(repoDir, remote, branch string) bool {
 	return err == nil
 }
 
+// GitRevParse resolves ref to a full object id in repoDir.
+func GitRevParse(repoDir, ref string) (string, error) {
+	return runGit(repoDir, "rev-parse", ref)
+}
+
+// GitAheadCount counts commits reachable from head that upstream does not hold.
+func GitAheadCount(repoDir, upstream, head string) (int, error) {
+	out, err := runGit(repoDir, "rev-list", "--count", upstream+".."+head)
+	if err != nil {
+		return 0, err
+	}
+	n, err := strconv.Atoi(strings.TrimSpace(out))
+	if err != nil {
+		return 0, fmt.Errorf("parse rev-list count %q: %w", out, err)
+	}
+	return n, nil
+}
+
+// GitMergeFFOnly advances the branch checked out at dir to ref, failing rather
+// than writing a merge commit when the move is not a fast-forward.
+func GitMergeFFOnly(dir, ref string) error {
+	_, err := runGit(dir, "merge", "--ff-only", ref)
+	return err
+}
+
+// GitSetUpstream points branch at the remote-tracking ref upstream.
+func GitSetUpstream(repoDir, branch, upstream string) error {
+	_, err := runGit(repoDir, "branch", "--set-upstream-to="+upstream, branch)
+	return err
+}
+
 // GitIsDirty reports whether the checkout at dir has uncommitted or untracked
 // changes.
 func GitIsDirty(dir string) (bool, error) {