Use difft with git via GIT_EXTERNAL_DIFF instead of gitconfig

Sometimes tools don't pass --no-ext-diff and difft can be very slow
and/or break tools that don't expect its output.

gitconfig

@@ -23,7 +23,6 @@
 [commit]
     verbose = true
 [diff]
-    external = difft
     algorithm = histogram
     mnemonicprefix = true
     renames = copies

zshrc

@@ -85,6 +85,10 @@ else
     export VISUAL="vi"
 fi
 
+if is_interactive && command_exists difft; then
+    export GIT_EXTERNAL_DIFF="difft"
+fi
+
 if mac; then
     # Don't pollute tar archives with ._ files (Apple double files)
     export COPYFILE_DISABLE=true