[PATCH 4/4] 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% change 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(-)