fix: sync z-index for github tree and hover overlay (#278)

Closes https://github.com/sourcegraph/sourcegraph/issues/242.

Author: lukeautry

src/libs/code_intelligence/HoverOverlay.scss

@@ -1,4 +1,5 @@
 @import '../gitlab/style.scss';
+@import '../../global-styles/variables.scss';
 
 .hover-overlay-mount__phabricator {
     .hover-overlay {
@@ -18,3 +19,7 @@
         }
     }
 }
+
+.hover-overlay {
+    z-index: $default-z-index;
+}

src/libs/gitlab/style.scss

@@ -1,7 +1,5 @@
 .hover-overlay-mount__gitlab {
     .hover-overlay {
-        z-index: 999;
-
         code {
             background: none;
             color: unset;

src/shared/global-styles/variables.scss

@@ -0,0 +1 @@
+$default-z-index: 2000;

src/shared/tree/tree.scss

@@ -1,8 +1,10 @@
+@import '../global-styles/variables.scss';
+
 .tree-mount {
     position: fixed;
     top: 0;
     left: 0;
-    z-index: 2000;
+    z-index: $default-z-index - 1;
     width: 100%;
     pointer-events: none;
     height: 100%;