zsh: enable iterm2 integration This is the iterm2 integration script as of today. Squinting a bit, this looks like the version of the file from https://github.com/gnachman/iTerm2/commit/3d8173763e1e7f53c08a7da52fc92db4e036395a, which is the most recent edit to the file on github.

# This is broken into its own file on account of not being able to
# properly make opendiff-w bail when opendiff is unavailable.
[merge-tools]
filemerge.priority = 100
filemerge.executable = opendiff-w
filemerge.args = $local $other -ancestor $base -merge $output