contrib: remove git-diffall

The functionality of the "git diffall" script in contrib/ was
incorporated into "git difftool" when the --dir-diff option was added
in v1.7.11 (ca. June, 2012).  Once difftool learned those features,
the diffall script became obsolete.

The only difference in behavior is that when comparing to the working
tree, difftool copies any files modified by the user back to the
working tree when the diff tool exits.  "git diffall" required the
--copy-back option to do the same.  All other diffall options have the
same meaning in difftool.

Make life easier for people choosing a tool to use by removing the old
diffall script.  A pointer in the release notes should be enough to
help current users migrate.

Helped-by: Tim Henigan <[email protected]>
Signed-off-by: Jonathan Nieder <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: jrn