t7501-commit.sh: explicitly check that -F prevents invoking the editor

t7501-commit.sh: explicitly check that -F prevents invoking the editor

The "--signoff" test case in t7500-commit.sh was setting VISUAL while
using -F -, which indeed tested that the editor is not spawned with -F.
However, having it there was confusing, since there was no obvious reason
to the casual reader for it to be there.

This commits removes the setting of VISUAL from the --signoff test, and
adds in t7501-commit.sh a dedicated test case, where the rest of tests for
-F are.

Signed-off-by: Adeodato Simó <dato@net.com.org.es>
Okay-then-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>