fix(changes): Discard takes only the ticked files and blocks (#87)

The Changes panel's "Discard all" threw away every changed file, ticked or
not. It is now "Discard (n)": the same selection Commit sends — whole ticked
files as `paths`, a partly ticked file's ticked blocks as `hunks` — and is
disabled with nothing ticked. The server's `discard` accepts `hunks`
(`selectionPatch` in reverse, `git apply -R`), building every patch before
applying any so a stale selection undoes nothing.

Closes #87

server/__tests__/git-diff-ops.test.js

@@ -851,6 +851,51 @@ describe("runGitAction — one change block inside a hunk (issue #50)", () => {
     assert.equal(fs.readFileSync(path.join(LANE, "tail.txt"), "utf8"), "l1\nl2\nl3\nl4\nl5\nl6");
   });
 
+  it("discards only the ticked blocks and whole files, nothing unticked (issue #87)", async () => {
+    const edited = threeBlocks();
+    write(LANE, "keep.txt", "untouched by discard\n");
+    write(LANE, "drop.txt", "gone\n");
+    const [hunk] = await hunkOf();
+    await assert.rejects(
+      () =>
+        runGitAction(LANE, "discard", {
+          paths: ["drop.txt"],
+          hunks: { "near.txt": [{ hunk: hunk.header, blocks: [0, 2] }] },
+        }),
+      { code: "ECONFIRM" }
+    );
+    await runGitAction(LANE, "discard", {
+      paths: ["drop.txt"],
+      hunks: { "near.txt": [{ hunk: hunk.header, blocks: [0, 2] }] },
+      confirm: true,
+    });
+    const want = [...edited];
+    want.splice(11, 0, "line 11");
+    want.splice(3, 2, "line 4", "line 5");
+    assert.deepEqual(read(), [...want, ""]);
+    assert.equal(fs.existsSync(path.join(LANE, "drop.txt")), false);
+    assert.equal(fs.readFileSync(path.join(LANE, "keep.txt"), "utf8"), "untouched by discard\n");
+  });
+
+  it("discards ticked blocks alone, and refuses a stale one with nothing undone", async () => {
+    const edited = threeBlocks();
+    const [hunk] = await hunkOf();
+    await assert.rejects(
+      () =>
+        runGitAction(LANE, "discard", {
+          hunks: { "near.txt": [{ hunk: "@@ -1,2 +1,2 @@", blocks: [0] }] },
+          confirm: true,
+        }),
+      { code: "EHUNKGONE" }
+    );
+    assert.deepEqual(read(), [...edited, ""]);
+    await runGitAction(LANE, "discard", {
+      hunks: { "near.txt": [{ hunk: hunk.header, blocks: [1] }] },
+      confirm: true,
+    });
+    assert.deepEqual(read(), [...edited.filter((l) => l !== "INSERTED"), ""]);
+  });
+
   it("refuses a block the hunk does not have", async () => {
     threeBlocks();
     const [hunk] = await hunkOf();