feat(git): per-block conflict view with mine/theirs choices (#45)

Opening a conflicted file in the Changes panel now shows each conflict
block as its two sides, labelled from git's markers and tinted apart,
with Keep mine / Take theirs / Keep both per block (undoable, ^/v to
jump). Save and mark resolved writes the chosen text through the
whole-file editor's PUT /git/file (baseSha stale check) then runs
resolve done. The Merge conflicts group's buttons are text, and a
conflicted file is marked with a red ! in the Files tree.

Fix: ls-files --cached lists an unmerged path once per stage, so the
tree showed a conflicted file three times; lsFiles now dedupes.

client/src/lib/__tests__/conflicts.test.ts

@@ -0,0 +1,74 @@
+/**
+ * @file Tests for the merge-conflict parser behind the Changes panel's
+ * conflict view: plain and diff3 blocks, the two side labels, applying a
+ * choice per block, and refusing a file whose markers do not balance.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+import { describe, it, expect } from "vitest";
+import { parseConflicts, applyChoices } from "../conflicts";
+
+const FILE = [
+  "top",
+  "<<<<<<< HEAD",
+  "mine 1",
+  "mine 2",
+  "=======",
+  "theirs 1",
+  ">>>>>>> origin/main",
+  "middle",
+  "<<<<<<< HEAD",
+  "a",
+  "||||||| base",
+  "old",
+  "=======",
+  "b",
+  ">>>>>>> feat",
+  "end",
+];
+
+describe("parseConflicts", () => {
+  it("splits text from conflict blocks and reads both labels", () => {
+    const parts = parseConflicts(FILE)!;
+    expect(parts.map((p) => p.kind)).toEqual(["text", "conflict", "text", "conflict", "text"]);
+    expect(parts[1]).toMatchObject({
+      ours: ["mine 1", "mine 2"],
+      theirs: ["theirs 1"],
+      oursLabel: "HEAD",
+      theirsLabel: "origin/main",
+    });
+    // diff3: the base section is not either side.
+    expect(parts[3]).toMatchObject({ ours: ["a"], theirs: ["b"], theirsLabel: "feat" });
+  });
+
+  it("returns an empty list of blocks for a file without markers", () => {
+    expect(parseConflicts(["a", "b"])).toEqual([{ kind: "text", lines: ["a", "b"] }]);
+  });
+
+  it("refuses markers that do not balance", () => {
+    expect(parseConflicts(["<<<<<<< HEAD", "x", "======="])).toBeNull();
+    expect(parseConflicts(["x", ">>>>>>> feat"])).toBeNull();
+  });
+});
+
+describe("applyChoices", () => {
+  it("writes each block as the chosen side, both sides in order for 'both'", () => {
+    const parts = parseConflicts(FILE)!;
+    expect(applyChoices(parts, ["ours", "both"])).toEqual([
+      "top",
+      "mine 1",
+      "mine 2",
+      "middle",
+      "a",
+      "b",
+      "end",
+    ]);
+    expect(applyChoices(parts, ["theirs", "theirs"])).toEqual([
+      "top",
+      "theirs 1",
+      "middle",
+      "b",
+      "end",
+    ]);
+  });
+});

client/src/lib/conflicts.ts

@@ -0,0 +1,87 @@
+/**
+ * @file Parses git's merge-conflict markers out of a file's lines, and writes
+ * the file back with one side (or both) chosen per block — the model behind
+ * the Changes panel's conflict view. Handles diff3 style (`|||||||` base
+ * section, dropped: it is neither side). Pure, no React.
+ * @author Nguyễn Ngọc Trí Vĩ <vinnt@smartgift.vn>
+ */
+
+export type ConflictChoice = "ours" | "theirs" | "both";
+
+export type ConflictPart =
+  | { kind: "text"; lines: string[] }
+  | { kind: "conflict"; ours: string[]; theirs: string[]; oursLabel: string; theirsLabel: string };
+
+const OPEN = /^<{7}(?: (.*))?$/;
+const BASE = /^\|{7}(?: .*)?$/;
+const MID = /^={7}$/;
+const CLOSE = /^>{7}(?: (.*))?$/;
+
+/**
+ * The file as text runs and conflict blocks, or null when its markers do not
+ * balance (a hand edit left half a block) — the caller falls back to the
+ * plain editor then rather than guess.
+ */
+export function parseConflicts(lines: string[]): ConflictPart[] | null {
+  const parts: ConflictPart[] = [];
+  let text: string[] = [];
+  let block: { ours: string[]; theirs: string[]; oursLabel: string } | null = null;
+  let section: "ours" | "base" | "theirs" = "ours";
+
+  for (const line of lines) {
+    if (!block) {
+      const open = line.match(OPEN);
+      if (open) {
+        if (text.length) parts.push({ kind: "text", lines: text });
+        text = [];
+        block = { ours: [], theirs: [], oursLabel: open[1] ?? "" };
+        section = "ours";
+      } else if (CLOSE.test(line) || MID.test(line)) {
+        return null;
+      } else {
+        text.push(line);
+      }
+      continue;
+    }
+    if (section === "ours" && BASE.test(line)) section = "base";
+    else if (section !== "theirs" && MID.test(line)) section = "theirs";
+    else if (section === "theirs" && CLOSE.test(line)) {
+      const close = line.match(CLOSE)!;
+      parts.push({ kind: "conflict", ...block, theirsLabel: close[1] ?? "" });
+      block = null;
+    } else if (OPEN.test(line)) return null;
+    else if (section === "ours") block.ours.push(line);
+    else if (section === "theirs") block.theirs.push(line);
+  }
+  if (block) return null;
+  if (text.length || !parts.length) parts.push({ kind: "text", lines: text });
+  return parts;
+}
+
+/** The file's lines with block `i` replaced by `choices[i]`; an unchosen block keeps its markers. */
+export function applyChoices(
+  parts: ConflictPart[],
+  choices: (ConflictChoice | undefined)[]
+): string[] {
+  const out: string[] = [];
+  let i = 0;
+  for (const p of parts) {
+    if (p.kind === "text") {
+      out.push(...p.lines);
+      continue;
+    }
+    const c = choices[i++];
+    if (c === "ours") out.push(...p.ours);
+    else if (c === "theirs") out.push(...p.theirs);
+    else if (c === "both") out.push(...p.ours, ...p.theirs);
+    else
+      out.push(
+        `<<<<<<< ${p.oursLabel}`.trimEnd(),
+        ...p.ours,
+        "=======",
+        ...p.theirs,
+        `>>>>>>> ${p.theirsLabel}`.trimEnd()
+      );
+  }
+  return out;
+}