Improvements to content history (#17746)

* Improvements to content history

* initialize content history when making an edit to an old item created before the introduction of content history
* show edit history for code comments on pull request files tab

* Fix a flaw in keepLimitedContentHistory
Fix a flaw in keepLimitedContentHistory, the first and the last should never be deleted

* Remove obsolete eager initialization of content history

templates/repo/pulls/files.tmpl

@@ -1,4 +1,8 @@
 {{template "base/head" .}}
+
+<input type="hidden" id="repolink" value="{{$.RepoRelPath}}">
+<input type="hidden" id="issueIndex" value="{{.Issue.Index}}"/>
+
 <div class="page-content repository view issue pull files diff">
 	{{template "repo/header" .}}
 	<div class="ui container {{if .IsSplitStyle}}fluid padded{{end}}">