Get correct absolute path of changed files when .changeset/ is not in the root of the repository

.changeset/sharp-tools-yell.md

@@ -0,0 +1,5 @@
+---
+"@changesets/cli": patch
+---
+
+`changeset add` and `changeset status` should now properly handle the situation where Changesets are managed from a directory different than the root of the repository.

.changeset/silly-sloths-brush.md

@@ -0,0 +1,5 @@
+---
+"@changesets/git": patch
+---
+
+`getChangedFilesSince` and `getChangedPackagesSinceRef` will now return the correct absolute paths of the changed files when the passed `cwd` is different from the repository's root.

packages/git/src/index.test.ts

@@ -1,3 +1,4 @@
+import path from "path";
 import fixtures from "fixturez";
 import spawn from "spawndamnit";
 import fileUrl from "file-url";
@@ -416,6 +417,39 @@ describe("git", () => {
         "packages/pkg-b/package.json"
       );
     });
+    it("should get correct full paths of changed files irrespective of cwd", async () => {
+      const ref = await spawn("git", ["rev-parse", "HEAD"], { cwd });
+
+      await add("packages/pkg-a/index.js", cwd);
+      await commit("Add packageA index", cwd);
+
+      await add("packages/pkg-b/index.js", cwd);
+      await commit("Added packageB index", cwd);
+
+      const filesChangedSinceRef = await getChangedFilesSince({
+        ref: ref.stdout.toString().trim(),
+        cwd,
+        fullPath: true
+      });
+      expect(filesChangedSinceRef[0]).toBe(
+        path.resolve(cwd, "packages/pkg-a/index.js")
+      );
+      expect(filesChangedSinceRef[1]).toBe(
+        path.resolve(cwd, "packages/pkg-b/index.js")
+      );
+
+      const filesChangedSinceRef2 = await getChangedFilesSince({
+        ref: ref.stdout.toString().trim(),
+        cwd: path.resolve(cwd, "packages"),
+        fullPath: true
+      });
+      expect(filesChangedSinceRef2[0]).toBe(
+        path.resolve(cwd, "packages/pkg-a/index.js")
+      );
+      expect(filesChangedSinceRef2[1]).toBe(
+        path.resolve(cwd, "packages/pkg-b/index.js")
+      );
+    });
   });
 
   describe("getChangedPackagesSinceRef", () => {

packages/git/src/index.ts

@@ -182,6 +182,22 @@ export async function isRepoShallow({ cwd }: { cwd: string }) {
 export async function deepenCloneBy({ by, cwd }: { by: number; cwd: string }) {
   await spawn("git", ["fetch", `--deepen=${by}`], { cwd });
 }
+async function getRepoRoot({ cwd }: { cwd: string }) {
+  const { stdout, code, stderr } = await spawn(
+    "git",
+    ["rev-parse", "--show-toplevel"],
+    { cwd }
+  );
+
+  if (code !== 0) {
+    throw new Error(stderr.toString());
+  }
+
+  return stdout
+    .toString()
+    .trim()
+    .replace(/\n|\r/g, "");
+}
 
 export async function getChangedFilesSince({
   cwd,
@@ -207,7 +223,9 @@ export async function getChangedFilesSince({
     .split("\n")
     .filter(a => a);
   if (!fullPath) return files;
-  return files.map(file => path.resolve(cwd, file));
+
+  const repoRoot = await getRepoRoot({ cwd });
+  return files.map(file => path.resolve(repoRoot, file));
 }
 
 // below are less generic functions that we use in combination with other things we are doing