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Backport #20346

Although `WalkGitLog` tries to test for `context.DeadlineExceededErr`
there is a small chance that the error will propagate to the reader
before it is recognised. This will cause the error to propagate up to
`renderDirectoryFiles` and cause a http status 500.

Here we check that the error passed is a `DeadlineExceededErr` via error.Is

Fix #20329
Signed-off-by: Andrew Thornton <art27@cantab.net>
Co-authored-by: zeripath <art27@cantab.net>