Fix sharing wizard page "Select Git Repository Location"

Fix sharing wizard page "Select Git Repository Location"

Enable selection of alternative repository mappings and highlight the selected
row fully. Selection was not possible since treeItem didn't show selectable text
in first column for alternative repository mappings (bug 296807).

Re-establish multi-selection by displaying most likely mapping on tree top level
and avoiding tree if only one git dir found (bug 297139).

Fix rendering of git directory in column Repository
  - on Windows the relative path was prefixed with the device name
    e.g. C:.git instead of .git
  - on Windows backslash was used as separator before .git
    which is inconsistent:
      e.g. src/org\.git instead of src/org/.git

Bug: 296807, 297139
Change-Id: Ie64d9e82609a62e4a117ed9e35156837b5d7b5b2
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>
Signed-off-by: Stefan Lay <stefan.lay@sap.com>