initial version of git icshow

[juchem]

Updated README.md with instructions.

git show doesn't have an option to set the external diff command other than the GIT_EXTERNAL_DIFF environment variable. Parameters order is quite strict. Although icdiff.py could be given a new option that accepts GIT_EXTERNAL_DIFF arguments, that's not my call to make and would greatly complicate this PR.

icdiff-git-extdiff is a dispatcher that converts GIT_EXTERNAL_DIFF parameters into icdiff arguments.

icdiff-git-extdiff uses sh -c as a driver so that icdiff.options can be set to something like --cols=$(tput cols). Currently, git-icdiff already plays well with such settings because of how the ICDIFF_OPTIONS variable is expanded when passed to git difftool --extcmd=.