[PATCH v2 9/9] core.abbrev: raise the default abbreviation to 12 hexdigits

As Peff said, responding in a thread started by Linus's suggestion to raise the default abbreviation to 12 hexdigits:

    I actually think "12" might be sane for a long time. That's 48
    bits of sha1, so we'd expect a 50% chance of a single collision
    at 2^24, or 16 million.  The biggest repository I know about (in
    number of objects) is the one holding all of the objects for all
    of the forks of torvalds/linux on GitHub. It's at about 15
    million objects.

    Which seems close, but remember that's the size where we expect
    to see a single collision. They don't become common until much
    later (I didn't compute an exact number, but Linus's 16x sounds
    about right). I know that the growth of the kernel isn't really
    linear, but I think the need to bump to "13" might not just be
    decades, but possibly a century or more.

    So 12 seems reasonable, and the only downside for it (or for "13", for
    that matter) is a few extra bytes. I dunno, maybe people will really
    hate that, but I have a feeling these are mostly cut-and-pasted anyway.

And this does exactly that.

Keep the tests working by explicitly asking for the old 7 hexdigits setting in the fake system-wide configuration file used for tests.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 environment.c        | 2 +-
 t/gitconfig-for-test | 3 +++
 2 files changed, 4 insertions(+), 1 deletion(-)