Add support for special symref FETCH_HEAD and MERGE_HEAD

Add support for special symref FETCH_HEAD and MERGE_HEAD

The RefDirectory class was not returning FETCH_HEAD and
MERGE_HEAD when trying to get all refs via getRefs(RefDatabase.ALL).
This fix adds constants for FETCH_HEAD and ORIG_HEAD and adds a
new getter getAdditionalRefs() to get these additional refs.
To be compatible with c git the getRefs(ALL) method will not return
FETCH_HEAD, MERGE_HEAD and ORIG_HEAD.

Change-Id: Ie114ca92e9d5e7d61d892f4413ade65acdc08c32
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>