support large packfiles with index v2

Grit has known about the "v2" pack index format for a while.
However, it never actually handled the extended offsets that
we get when indexing packfiles that are larger than 2
gigabytes.

When an object is at an offset smaller than 2G, its byte
offset into the packfile is placed in the first table of
4-byte offset values. If it's past that, then the MSB is set
on its offset in the 4-byte table, and the rest of the
4-byte integer specifies an offset into an 8-byte table that
follows.

With this patch, grit should handle arbitrarily large packs
(limited only by the pack format itself).

A few notes on the patch itself:

  - I unpack using two "N" formats instead of "Q>", because
    "Q>" is not available in ruby < 1.9.3

  - No automated test is included, because you need a
    packfile that is greater than 2G. I did test it by hand.

Some more tweaks here:

	- Do not use `strftime`, because it's not assured
	to be cross-platform

	- Use C-like string formatting for Great Glory
	When Printing Numbers.

	- Always print an email address -- even if we don't
	have one. A missing email field will crash `fsck`.

This shouldn't technically be allowed but we've seen a few cases of
it in existing repositories on github.com so let's just deal with
it.

Fix typo in tree method doc

This lets us pass an :env so we can use GIT_ALTERNATE_OBJECT_DIRECTORIES
to check if a commit applies across repositories.

Otherwise, the git-rev-parse will return whatever is given as an arg
when the ref doesn't exist. e.g.,

  $ git rev-parse some-bad-ref
  some-bad-ref
  fatal: ambiguous argument 'some-bad-ref': unknown revision or path not in the working tree.

The error message is on stderr and git-rev-parse exits with non-zero
but the ref name is still output.

The problem here is that code often calls rev_list like:

    git.rev_list({}, "some-bad-ref")

Then rev_list tries to convert some-bad-ref to a SHA1, gets back the
ref string, but continues on anyway. This eventually results in the
rev_list failing to look up the object because it assumes its a SHA1
when its really a ref string.

Fix warnings on 1.9

Fix the gemspec

Fix for files with leading or trailing spaces

Default parameter for with_timeout incorrect