Prevent context deadline error propagation in GetCommitsInfo (#20346) (#20361)

Prevent context deadline error propagation in GetCommitsInfo (#20346) (#20361)

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>