Fix missing packed-refs entries during ref lookup/enumeration
Some codepaths in RefDatabase allowed the caller to lookup a ref
without making sure the packed-refs file was loaded into memory.
This caused refs which were only packed to appear to not exist,
which meant fetch would still try to fetch something we already
have had for quite some time.
This resolves the issue I was seeing where `jgit fetch` was always
downloading the stable branch of egit.git, even though it has not
changed in many weeks.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>