Simplify JGit isBareRepository implementation

Most recent implementation attempted to match the structure of the
CliGitAPI implementation, without concern for code duplication.  This
small refactoring reduces code duplication and reduces the chance that
a repository will remain open due to an exception in the
isBareRepository method.
Add isBareRepository test case for a non-existent file inside a git repo

The CliGit implementation uses the command:

git --git-dir=arg1 rev-parse --is-bare-repository

That throws an exception if a non-existent file is passed as arg1.

The JGit implementation attempts to match the CliGit implementation
but doesn't quite get it.

The Javadoc for the CliGit implementation at one time stated that arg1
must be the ".git" directory.  If the caller passes the .git directory
(for a working directory) or the root directory of the repository (for
a bare repository), then the JGit and CliGit implementations return
consistent results.
Reorganize isBareRepository assertions by priority

The assertions are intended to detect unintentional changes to a
deprecated API.  It is safe to assume that since the API is
deprecated, its behavior should not change with future plugin
releases.
Merge branch 'master-assess-isBareRepository' into master-work

Conflicts:
	src/test/java/org/jenkinsci/plugins/gitclient/GitAPITestCase.java