GC should not pack objects only referenced by ORIG_HEAD,...

GC should not pack objects only referenced by ORIG_HEAD,...

There are references which are returned by
RefDatabase.getAdditionalRefs() which are allowed to point to
non-existing objects. These refs should not save objects from being
garbage collected. Examples for these references are ORIG_HEAD,
MERGE_HEAD, FETCH_HEAD and CHERRY_PICK_HEAD. Native git will not take
these references into account when doing a gc and therefore these
references may point to non-existing objects after a gc. Teach JGit's
GC to behave the same: ignore additional refs if they don't start with
"refs/". Examples for refs returned by getAdditionalRefs() which do
start with "refs/" are the bootstrap refs when using reftree's (see
commit 115f1ad3974d1162b33d1c8eff466019d1f249f3). See also
http://article.gmane.org/gmane.comp.version-control.git/294126.

Bug: 479697
Change-Id: I10e40589f13e72aacdd9f86f3b44696fd1cd068a
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>