A group of updates can be applied by updating the tree in one step,
writing out a new root tree, and storing its SHA-1. If references
are stored in RefTrees, comparing two repositories is a matter of
checking if two SHA-1s are identical. Without RefTrees comparing two
repositories requires listing all references and comparing the sets.
Track the "refs/" directory as a root tree by storing references
that point directly at an object as a GITLINK entry in the tree.
For example "refs/heads/master" is written as "heads/master".
Annotated tags also store their peeled value with ^{} suffix, using
"tags/v1.0" and "tags/v1.0^{}" GITLINK entries.
Symbolic references are written as SYMLINK entries with the blob of
the symlink carrying the name of the symbolic reference target.
HEAD is outside of "refs/" namespace so it is stored as a special
"..HEAD" entry. This name is chosen because ".." is not valid in
a reference name and it almost looks like "../HEAD" which names
HEAD if the reader was inside of the "refs/" directory.
A new Command type is required to handle symbolic references and
peeled references.
Change-Id: Id47e5d4d32149a9e500854147edd7d93c1041a39
73eb32be82