t7510: invoke git as part of &&-chain

If `git commit-tree HEAD^{tree}` fails on us and produces no output on stdout, we will substitute that empty string and execute `git tag ninth-unsigned`, i.e., we will tag HEAD rather than a newly created object. But we are lucky: we have a signature on HEAD, so we should eventually fail the next test, where we verify that "ninth-unsigned" is indeed unsigned. We have a similar problem a few lines later. If `git commit-tree -S` fails with no output, we will happily tag HEAD as "tenth-signed". Here, we are not so lucky. The tag ends up on the same commit as "eighth-signed-alt", and that's a signed commit, so t7510-signed-commit will pass, despite `git commit-tree -S` failing. Make these `git commit-tree` invocations a direct part of the &&-chain, so that we can rely less on luck and set a better example for future tests modeled after this one. Fix a 9/10 copy/paste error while at it. Signed-off-by: Martin Ågren <martin.agren@gmail.com> Signed-off-by: Brandon Richardson <brandon1024.br@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>