pkg/vcs: parse git diffs

Provide a functionality to extract the files affected by a git patch.

pkg/vcs/git.go

@@ -654,3 +654,14 @@ func (git *git) PushCommit(repo, commit string) error {
 	}
 	return nil
 }
+
+var fileNameRe = regexp.MustCompile(`(?m)^diff.* b\/([^\s]+)`)
+
+// ParseGitDiff extracts the files modified in the git patch.
+func ParseGitDiff(patch []byte) []string {
+	var files []string
+	for _, match := range fileNameRe.FindAllStringSubmatch(string(patch), -1) {
+		files = append(files, match[1])
+	}
+	return files
+}

pkg/vcs/git_test.go

@@ -462,3 +462,25 @@ func TestGitFetchShortHash(t *testing.T) {
 	_, err := local.CheckoutCommit(remoteRepoDir, refCommit.Hash[:12])
 	assert.NoError(t, err)
 }
+
+func TestParseGitDiff(t *testing.T) {
+	files := ParseGitDiff([]byte(`diff --git a/a.txt b/a.txt
+index 4c5fd91..8fe1e32 100644
+--- a/a.txt
++++ b/a.txt
+@@ -1 +1 @@
+-First file
++First file!
+diff --git a/b.txt b/b.txt
+new file mode 100644
+index 0000000..f8a9677
+--- /dev/null
++++ b/b.txt
+@@ -0,0 +1 @@
++Second file.
+diff --git a/c/c.txt b/c/c.txt
+new file mode 100644
+index 0000000..e69de29
+`))
+	assert.ElementsMatch(t, files, []string{"a.txt", "b.txt", "c/c.txt"})
+}