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/lib/git-diff.js

@@ -1241,16 +1241,18 @@ async function diffHunks(dir, file, context) {
  * in file order, to be applied FORWARD onto an index holding HEAD. Every
  * header is matched against a fresh diff like `hunkPatch`'s; null when one is
  * gone or names a block the hunk lacks. Each hunk's new-side start is left as
- * git wrote it — `git apply` locates a hunk by its old side.
+ * git wrote it — `git apply` locates a hunk by its old side. `reverse` cuts
+ * it for `apply -R` off the working tree instead (the partial discard, #87),
+ * where the NEW side is the one checked and every start stays git's own.
  */
-async function selectionPatch(dir, file, context, picks) {
+async function selectionPatch(dir, file, context, picks, reverse = false) {
   const diff = await diffHunks(dir, file, context);
   if (!diff || !picks.length) return null;
   const at = (header) => diff.hunks.findIndex((h) => h.slice(0, h.indexOf("\n")) === header);
   if (picks.some((p) => at(p.hunk) < 0)) return null;
   const bodies = [];
   for (const p of [...picks].sort((x, y) => at(x.hunk) - at(y.hunk))) {
-    const body = onlyBlock(diff.hunks[at(p.hunk)], p.blocks, false);
+    const body = onlyBlock(diff.hunks[at(p.hunk)], p.blocks, reverse);
     if (!body) return null;
     bodies.push(body);
   }