[diffoscope] diffoscope with git difftool

[bnewbold at robocracy.org]

Hey Folks!

I learned a new way to use diffoscope today: for visualizing diffs with 
git. This is only really helpful if you happen to have binary files (eg, 
.pdf, executables) or archives (.tar.gz, etc) in git, which some consider 
faux pas, but happens pretty frequently.

If you add something like this to your ~/.gitconfig:

   [difftool]
     prompt = false
   [difftool "diffoscope"]
     cmd = "if [ $LOCAL = /dev/null ]; then diffoscope --new $REMOTE; else diffoscope $LOCAL $REMOTE; fi"

Then you can can replace `git diff <args>` with `git difftool -t 
diffoscope <args>` and get console output. Additionally, if you install 
the `ruby-bcat` package on debian to get the "browser cat" tool (for 
viewing HTML piped into stdin), you can add:

   [difftool "diffoscope-html"]
     cmd = "if [ $LOCAL = /dev/null ]; then diffoscope --new $REMOTE --html 
- | bcat; else diffoscope $LOCAL $REMOTE --html - | bcat; fi"

and use `git difftool -t diffoscope-html <args>.

Neato!

It might be helpful in the future for diffoscope to recognize that if the 
first argument is /dev/null (or an empty file) that the `--new` argument 
is implied, but this might also be unexpected behavior. I worked around it 
with the above shell hack (git difftool passes /dev/null as the "$LOCAL" 
when a file is new vs. changed).

--bryan