https://en.wikibooks.org/wiki/Git/Internal_structure
A commit is reachable if it is pointed to by a branch, tag or reflog entry,
or is a parent of a commit which is reachable.
A tree is correspondingly reachable if it is pointed to by a reachable commit,
and a blob is reachable if it is pointed to by a reachable tree.
Other commit/tree/blob objects are unreachable,
and are not really serving any purpose beyond taking up space.

Interesting:

Types of git objects:
  blob (every file under source control is written into a blob)
  tree
  commit
  tag

Tree object:
- Header info
For each file/dir in the directory:
-- file permissions
-- object type (blob/tree)
-- sha-1
-- file/dir name


alias deflate="perl -MCompress::ZLib -e 'undef $/; print uncompress(<>)'"

git cat-file -t <hash>  get type
             -p <hash>  print

100644 blob <sha-1>1   .gitignore
100644 blob <sha-1>2   README.markdown
100644 blob <sha-1>2   TODO
100644 blob <sha-1>2   fix.txt


Commit (how git stores "snapshots") object:
- Author info
- Committer info
- Commit message (subjects)
- sha-1 of any parent commits (we can have multiple parents for branching/merging)
- sha-1 of the tree that the commit points to

???
git ls-tree


References:
- sha-1 hash
- branches
- HEAD
- Ancestry Refs (HEAD~, HEAD~2, HEAD~3, HEAD^, HEAD^2)

Branches:
- reference to the latest commit on that specific branch
  HEAD alias - reference to the latest commit on the current branch