t7005-editor.sh: Don't invoke real vi when it is in GIT_EXEC_PATH

t7005-editor.sh: Don't invoke real vi when it is in GIT_EXEC_PATH

The git wrapper executable always prepends the GIT_EXEC_PATH build
variable to the current PATH, so prepending "." to the PATH is not
enough to give precedence to the fake vi executable.

The --exec-path option allows to prepend a directory to PATH even before
GIT_EXEC_PATH (which is added anyway), so we can use that instead.

Signed-off-by: Björn Steinbrink <B.Steinbrink@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>