Make goto and open commit buttons icon-style

Summary:
The styling on these buttons changed recently-ish. They don't match the other ISL buttons and look out of place. On web it also looked unreadable in light themes due to a fg color mismatch.

Let's just use icon buttons for consistency.

Reviewed By: muirdm

Differential Revision: D116701714

fbshipit-source-id: ba9a3a86a9b7b4d5f65b7af9ef5b703b15602277

Author: evangrayk

addons/isl/src/Commit.tsx

@@ -655,6 +655,7 @@ export const Commit = memo(
             )}
             delayMs={250}>
             <Button
+              icon
               aria-label={t('Go to commit "$title"', {replace: {$title: commit.title}})}
               className={css.gotoButton}
               onClick={async event => {

addons/isl/src/CommitTreeList.css

@@ -305,11 +305,6 @@
   transition: opacity 0.1s;
 }
 
-.goto-button button,
-.open-commit-info-button {
-  background-color: var(--background);
-}
-
 .commit:hover .goto-button,
 .commit:hover .open-commit-info-button {
   opacity: 1;