t1006: extract helper functions into new 'lib-cat-file.sh'

t1006: extract helper functions into new 'lib-cat-file.sh'

Extract utility functions from the cat-file's test script
't1006-cat-file.sh' into a new 'lib-cat-file.sh' dedicated library file.

A subsequent commit will need these functions. This improves the code
reuse and readability, enabling future cat-file tests to share these
helpers without duplicating code.

While at it update the style of this line to follow coding
guidelines:

. "$TEST_DIRECTORY/lib-loose.sh"

to

. "$TEST_DIRECTORY"/lib-loose.sh

Signed-off-by: Eric Ju <eric.peijian@gmail.com>
Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>