Prevent a JS error in MergeRequestTabs

When `window.location.hash` is pointing to a note, e.g. `#note_1234`,
`scrollToElement` would throw an error because a selector such as
`.commits #note_1234` doesn't exist, so `offset()` returned `undefined`.

This error would prevent subsequent calls from running, which caused the
loading spinner to never be hidden.

Now we ensure the selector returns a valid element before trying to
scroll to it.

app/assets/javascripts/merge_request_tabs.js.coffee

@@ -68,8 +68,8 @@ class @MergeRequestTabs
 
   scrollToElement: (container) ->
     if window.location.hash
-      top = $(container + " " + window.location.hash).offset().top
-      $('body').scrollTo(top)
+      $el = $("#{container} #{window.location.hash}")
+      $('body').scrollTo($el.offset().top) if $el.length
 
   # Activate a tab based on the current action
   activateTab: (action) ->
@@ -127,7 +127,7 @@ class @MergeRequestTabs
         document.getElementById('commits').innerHTML = data.html
         $('.js-timeago').timeago()
         @commitsLoaded = true
-        @scrollToElement(".commits")
+        @scrollToElement("#commits")
 
   loadDiff: (source) ->
     return if @diffsLoaded
@@ -137,7 +137,7 @@ class @MergeRequestTabs
       success: (data) =>
         document.getElementById('diffs').innerHTML = data.html
         @diffsLoaded = true
-        @scrollToElement(".diffs")
+        @scrollToElement("#diffs")
 
   # Show or hide the loading spinner
   #