After overwriting a tag with a new tag, "Reread references" action
in "gitk" correctly moved the marker in the display, but it failed
to discard a cached contents of the tag (even "Reload" didn't).
* da/gitk-reload-tag-contents:
gitk: Rename 'tagcontents' to 'cached_tagcontent'
gitk: Teach "Reread references" to reload tags
gitk: Avoid Meta1-F5