t4202: refactor test

t4202: refactor test

Subsequent patches will want to reuse the 'signed' branch that the
'log --graph --show-signature' test creates and uses.

Split the set-up part into a test of its own, and make the existing
test into a separate one that only inspects the history on the 'signed'
branch. This way, it becomes clearer that tests added by subsequent
patches reuse the 'signed' branch in the same way.

Signed-off-by: Mehul Jain <redacted>
Signed-off-by: Junio C Hamano <redacted>