glossary: add "commit graph" description

Git has an additional "commit graph" capability that supplements the
normal commit object's directed acyclic graph (DAG). The supplemental
commit graph file is designed for speed of access.

Describe the commit graph both from the normative DAG view point and
from the commit graph file perspective.

Also, clarify the link between the branch ref and branch tip
by linking to the `ref` glossary entry, matching this commit graph
entry.

The commit-graph file is also distinguished by its hyphenation.

Subsequent commit catches the few cases where the hyphenation of
commit-graph was missing.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

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