t5305: use "git -C"

t5305: use "git -C"

This test unpacks objects into a separate repository, and
accesses it by setting GIT_DIR in a subshell. We can do the
same thing these days by using "git init <repo>" and "git
-C". In most cases this is shorter, though when there are
multiple commands, we may end up repeating the "-C".

However, this repetition can actually be a good thing. This
patch also fixes a bug introduced by 512477b (tests: use
"env" to run commands with temporary env-var settings,
2014-03-18). That commit essentially converted:

   (GIT_DIR=...; export GIT_DIR
    cmd1 &&
    cmd2)

into:

   (GIT_DIR=... cmd1 &&
    cmd2)

which obviously loses the GIT_DIR setting for cmd2 (we never
noticed the bug because it simply runs "cmd2" in the parent
repo, which means we were simply failing to test anything
interesting). By using "git -C" rather than a subshell, it
becomes quite obvious where each command is supposed to be
running.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>