Backport #23219
When fetching remotes for conflict checking, skip unnecessary and
potentially slow writing of commit graphs.
In a test with the Blender repository, this reduces conflict checking
time for one pull request from about 2s to 0.1s.
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Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: Brecht Van Lommel <brecht@blender.org>