ci: skip GitHub workflow runs for already-tested commits/trees

When pushing a commit that has already passed a CI or PR build
successfully, it makes sense to save some energy and time and skip the
new build.

Let's teach our GitHub workflow to do that.

For good measure, we also compare the tree ID, which is what we actually
test (the commit ID might have changed due to a reworded commit message,
which should not affect the outcome of the run).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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