Improve diff detection

This fixes some things on pages displaying diffs by improving the detection of diffs
and exposing a single method to check if the page has a diff.

- Compare pages now have a detection as well:
  Use pageDetect.isCompare() to detect: /<owner>/<repo>/compare/<revision-range>

- Exposed a new simple pageDetect.hasDiff() method to detect if there is a diff on a page.
  It combines commits, PR diffs, compare pages etc.

- Diff signs are now removed on compare pages as well.

extension/content.js

@@ -383,7 +383,7 @@ document.addEventListener('DOMContentLoaded', () => {
 				addPatchDiffLinks();
 			}
 
-			if (pageDetect.isCommit() || pageDetect.isPR() || pageDetect.isPRFiles()) {
+			if (pageDetect.hasDiff()) {
 				removeDiffSigns();
 			}
 

extension/page-detect.js

@@ -29,6 +29,10 @@ window.pageDetect = (() => {
 
 	const isCommit = () => isSingleCommit() || isPRCommit() || (isPRFiles() && $('.full-commit').length > 0);
 
+	const isCompare = () => isRepo() && /^\/compare\//.test(getRepoPath());
+
+	const hasDiff = () => isRepo() && (isSingleCommit() || isPRCommit() || isPRFiles() || isCompare() || (isPR() && $('.diff-table').length > 0));
+
 	const isReleases = () => isRepo() && /^\/(releases|tags)/.test(getRepoPath());
 
 	const isBlame = () => isRepo() && /^\/blame\//.test(getRepoPath());
@@ -65,6 +69,8 @@ window.pageDetect = (() => {
 		isCommitList,
 		isSingleCommit,
 		isCommit,
+		isCompare,
+		hasDiff,
 		isReleases,
 		isBlame,
 		isNotifications,