commit-graph: add 'verify' subcommand

If the commit-graph file becomes corrupt, we need a way to verify
that its contents match the object database. In the manner of
'git fsck' we will implement a 'git commit-graph verify' subcommand
to report all issues with the file.

Add the 'verify' subcommand to the 'commit-graph' builtin and its
documentation. The subcommand is currently a no-op except for
loading the commit-graph into memory, which may trigger run-time
errors that would be caught by normal use. Add a simple test that
ensures the command returns a zero error code.

If no commit-graph file exists, this is an acceptable state. Do
not report any errors.

Helped-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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