Fix various trivial problems (#32861)

1. add/improve comments to help future readers could understand the
problem more easily.
2. add an error log to LDAP with username fallback
3. use `or` instead of `Iif` for "repo/branch_dropdown" (`Iif` was a
mistake, but it doesn't really affect the UI)
4. add `tw-font-mono` style to container digest to match dockerhub
5. fix a bug in RepoBranchTagSelector: the form is not updated when
there is no click to an item

---------

Co-authored-by: delvh <dev.lh@web.de>

templates/repo/commit_page.tmpl

@@ -68,11 +68,13 @@
 											<p id="cherry-pick-content" class="branch-dropdown"></p>
 
 											<form method="get">
+												{{/*FIXME: CurrentRefShortName seems not making sense here (old code),
+												because the "commit page" has no "$.BranchName" info, so only using DefaultBranch should be enough */}}
 												{{template "repo/branch_dropdown" dict
 													"Repository" .Repository
 													"ShowTabBranches" true
 													"CurrentRefType" "branch"
-													"CurrentRefShortName" (Iif $.BranchName $.Repository.DefaultBranch)
+													"CurrentRefShortName" (or $.BranchName $.Repository.DefaultBranch)
 													"RefFormActionTemplate" (print "{RepoLink}/_cherrypick/" .CommitID "/{RefShortName}")
 												}}
 												<input type="hidden" id="cherry-pick-type" name="cherry-pick-type"><br>