difftool: implement the functionality in the builtin

This patch gives life to the skeleton added in the previous patch.

The motivation for converting the difftool is that Perl scripts are not at
all native on Windows, and that `git difftool` therefore is pretty slow on
that platform, when there is no good reason for it to be slow.

In addition, Perl does not really have access to Git's internals. That
means that any script will always have to jump through unnecessary
hoops.

The current version of the builtin difftool does not, however, make full
use of the internals but instead chooses to spawn a couple of Git
processes, still, to make for an easier conversion. There remains a lot
of room for improvement, left for a later date.

Note: to play it safe, the original difftool is still called unless the
config setting difftool.useBuiltin is set to true.

The reason: this new, experimental, builtin difftool will be shipped as
part of Git for Windows v2.11.0, to allow for easier large-scale
testing, but of course as an opt-in feature.

Sadly, the speedup is more noticable on Linux than on Windows: a quick
test shows that t7800-difftool.sh runs in (2.183s/0.052s/0.108s)
(real/user/sys) in a Linux VM, down from  (6.529s/3.112s/0.644s), while
on Windows, it is (36.064s/2.730s/7.194s), down from
(47.637s/2.407s/6.863s). The culprit is most likely the overhead
incurred from *still* having to shell out to mergetool-lib.sh and
difftool--helper.sh.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho