RepositoryModel will use String rather than RefModel to track the current
symbolic head and available heads.

Added convenience methods to JGitUtils to support retrieving available
heads as List<String>.
When resolving the symbolic head target as a String, if the head is
detached, attempt to match the commit SHA1 against the known tags, using
the most recent tag if more than one matches.
Revised error messaging to better reflect actual outcome.
Adjusted tab indexes on edit repository page to include default head combo
box.
Updated message key for default head combo box to use uppercase "HEAD".