Add function to reverse a patch

src/patch/reverse.js

@@ -0,0 +1,25 @@
+export function reversePatch(structuredPatch) {
+  const result = {...structuredPatch, hunks: structuredPatch.hunks.slice()};
+  const { oldFileName, newFileName, oldHeader, newHeader, hunks } = result;
+
+  result.oldFileName = newFileName;
+  result.oldHeader = newHeader;
+  result.newFileName = oldFileName;
+  result.newHeader = oldHeader;
+
+  for (const hunk of hunks) {
+    const { oldLines, oldStart, newLines, newStart, lines } = hunk;
+    hunk.oldLines = newLines.slice();
+    hunk.oldStart = newStart;
+    hunk.newLines = oldLines.slice();
+    hunk.newStart = oldStart;
+
+    hunk.lines = lines.map(l => {
+      if (l.startsWith('-')) return `+${l.slice(1)}`;
+      if (l.startsWith('+')) return `-${l.slice(1)}`;
+      return l;
+    });
+  }
+
+  return structuredPatch;
+}