git-commit-graph.txt: refer to "*commit*-graph file"

git-commit-graph.txt: refer to "*commit*-graph file"

This document sometimes refers to the "commit-graph file" as just "the
graph file". This saves a couple of words here and there at the risk of
confusion. In particular, the documentation for `git commit-graph read`
appears to suggest that there are indeed different types of graph files.

Let's just write out the full name everywhere.

The full name, by the way, is not the dash-less "commit graph file".
Use the dashed form. (The next commit will fix the remaining few
instances of the "commit graph file" in this document.)

Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>