[RFC PATCH] mergetools: support difftool.tabbed setting

I was asked how to configure "git difftool" to open files using several
tabs and stop spawning diff application on every modified file. I looked
into Git source and found no possibility to run diff tool at one step.

The patch allows a user to view diffs in single window at one go. The
current implementation is still poor and it can be used solely for
demonstration purposes. To see it in action, tweak the local gitconfig:

    git config difftool.prompt false
    git config difftool.tabbed true

Then run:

    git difftool -t vimdiff

Or:

    git difftool -t meld

The solution has some restrictions, diffing up to ten files works now (I
did not bother with dynamic memory allocation), and it does not handle spaces
in file names (I do not know how to pass them correctly to underlying tools
without "xargs -0").

I think the git-difftool--helper should be changed so that it could
process many files in single invocation and it would not use a temporary
file by itself. A similar behaviour can be done in git-mergetool, too.

Do you have ideas how to better implement such a feature? Any comments
are welcome.

P.S.: I'm attaching screenshots for a clear demo what I mean.
---
 diff.c                |  4 ++--
 git-mergetool--lib.sh | 36 +++++++++++++++++++++++++++++++++++-
 mergetools/meld       |  4 ++++
 mergetools/vimdiff    | 17 +++++++++++++++++
 4 files changed, 58 insertions(+), 3 deletions(-)