Patch Set 4: I would prefer that you didn't submit this; Unclean IP, do not check in

We'll need a CQ for IP review since this change is way larger than 250 lines hence marking this as unclean IP. I'll create the CQ as soon as this change has settled and review is fine.

I played around with the UI, in general you are moving in the right direction :)

I think from usability perspective we need some polishing:
- the label for the tag name should be just "Tag name*:"
  more detailed descriptions should IMO go into the online help or a tooltip (don't know what the Eclipse UI guidelines recommend here)
- it would be useful to see the branch name the tag will be created on
- IMO the list of existing tags for the given branch should be always shown as another list on the left side of the dialog (probably this should be sorted in reverse chronological order, means latest on top). I didn't expect to find this list behind the "Details >>" button.
- when entering a tag name which already exists the tag list should scroll to this already existing tag and the tag name edit field background should turn red to make the user aware of the fact that this tag already exists. Maybe then a help text should appear "Tag xyz already exists, if you want to replace it check the force option".
- if an existing branch has been selected from the list there is no way to create a new tag anymore except closing and re-opening the dialog. This could be solved by adding a pseudo entry in the list which is named e.g. <create new tag>. Selecting this entry should then clear all the edit fields so that a new tag can be defined.
- I think the label "Overwrite existing tag" should be changed to "Force: replace existing tag" (see description of -f in [1], as long as this checkbox is not checked the data for an existing tag should be read-only. Then users will rather think twice before they change an existing tag.
- the advanced mode is cool :) I think the label should be a bit more to the point, e.g. "Select commit to tag". To notify the user that he can start typing display the little bulb icon in front of the edit field (used e.g. for the URI field of clone wizard you open via "Import > Git Repository") and display the explanation as a tooltip on this icon.

[1] http://www.kernel.org/pub/software/scm/git/docs/git-tag.html