Merge branch 'vr/use-our-perl-in-tests'

Some implementations of Perl terminates "lines" with CRLF even when
the script is operating on just a sequence of bytes.  Make sure to
use "$PERL_PATH", the version of Perl the user told Git to use, in
our tests to avoid unnecessary breakages in tests.

* vr/use-our-perl-in-tests:
t/README: add a bit more Don'ts
tests: enclose $PERL_PATH in double quotes
t/test-lib.sh: export PERL_PATH for use in scripts
t: Replace 'perl' by $PERL_PATH

diff --cc t/t5300-pack-object.sh
Simple merge

diff --cc t/t9300-fast-import.sh
Simple merge