t7500: fix GIT_EDITOR shell snippet

t7500: fix GIT_EDITOR shell snippet

2140b140 (commit: error out for missing commit message template,
2011-02-25) defined

    GIT_EDITOR="echo hello >\"\$1\""

for these two tests, with the intention that 'hello' would be
written in the given file, but as Phillip Wood points out,
GIT_EDITOR is invoked by shell after getting expanded to

    sh -c 'echo hello >"$1" "$@"' 'echo hello >"$1"' path/to/file

which is not what we want.

Signed-off-by: Junio C Hamano <gitster@pobox.com>