Add "--long" option to JGit describe

Add "--long" option to JGit describe

Native git supports "git describe --long". This will enforce returning a
long description of a commit even if a tag is directly pointing to the
commit (in contrast to just returning the tag name as it is now). This
commit teaches JGits DescribeCommand and the describe command in the pgm
package to support "--long".

Bug: 460991
Change-Id: I65e179b79e89049c6deced3c71cb3ebb08ed0a8f
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>