t7001: avoid using `cd` outside of subshells

t7001: avoid using `cd` outside of subshells

Avoid using `cd` outside of subshells since, if the test fails,
there is no guarantee that the current working directory is the
expected one, which may cause subsequent tests to run in the wrong
directory.

While at it, make some other tests more concise by replacing
simple subshells with `git -C`.

Signed-off-by: Shubham Verma <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>