fix(git): the branch switcher lists and checks out branches only the remote has

GET /api/lanes/:id/git/branches read only refs/heads, so a branch someone
else pushed never showed after a fetch or pull, and `switch` refused it
(ENOBRANCH). It now also returns `remotes`: fetched remote-tracking branches
with no local branch of that name (origin/HEAD left out), listed under
"On the remote"; switching to one runs `git switch -c <b> --track
refs/remotes/<remote>/<b>` with the remote ref looked up, never passed
through (bare name only when exactly one remote has it, else EAMBIGUOUS).
The bar also re-reads the branch list after a fetch or pull instead of
keeping the list it loaded first.

server/lib/git-ops.js

@@ -55,6 +55,7 @@ const {
   git: rawGit,
   isValidBranchName,
   listBranches,
+  listRemoteBranches,
   branchCheckedOutAt,
   statusCounts,
   operationInProgress,
@@ -532,7 +533,31 @@ async function runGitAction(dir, action, body = {}) {
     if (exists) throw bad("EBRANCHEXISTS", `branch ${branch} already exists`);
     return done(action, await git(dir, ["switch", "-c", branch]), branch);
   }
-  if (!exists) throw bad("ENOBRANCH", `branch ${branch} does not exist locally`);
+  if (!exists) {
+    // A branch only a remote has (fetched, never checked out here): its local
+    // tracking branch is created and switched to. Named either as the remote
+    // ref (`origin/feature`) or bare (`feature`, when exactly one remote has
+    // it). The remote ref is looked up among the fetched ones, never passed
+    // through from the request.
+    const remote = (await listRemoteBranches(dir)).filter(
+      (r) => r.name === branch || r.branch === branch
+    );
+    const pick = remote.find((r) => r.name === branch) || (remote.length === 1 ? remote[0] : null);
+    if (!pick) {
+      if (remote.length > 1) {
+        throw bad(
+          "EAMBIGUOUS",
+          `branch ${branch} is on several remotes (${remote.map((r) => r.name).join(", ")}): pick one`
+        );
+      }
+      throw bad("ENOBRANCH", `branch ${branch} does not exist locally or on a remote`);
+    }
+    return done(
+      action,
+      await git(dir, ["switch", "-c", pick.branch, "--track", `refs/remotes/${pick.name}`]),
+      pick.branch
+    );
+  }
   const busyAt = await branchCheckedOutAt(dir, branch);
   if (busyAt) throw bad("EBRANCHBUSY", `branch ${branch} is checked out at ${busyAt}`);
   return done(action, await git(dir, ["switch", branch]), branch);