contrib: add git-diffall script

contrib: add git-diffall script

The 'git difftool' allows the user to view diffs using an external tool.
It runs a separate instance of the tool for each file in the diff. This
makes it tedious to review changes spanning multiple files.

The 'git-diffall' script instead prepares temporary directories with the
files to be compared and launches a single instance of the external diff
tool to view them (i.e. a directory diff).

The 'diff.tool' or 'merge.tool' configuration variable is used to specify
which external tool is used.

Signed-off-by: Tim Henigan <tim.henigan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>