Gazelle: remove support for graphical multidiff (#357)
Gazelle: remove support for graphical multidiff (#357)

buildifier/differ supports both graphical and non-graphical
diff. The graphical diff doesn't work though: it generates a
command line for tkdiff, but tkdiff doesn't actually accept commands
in that format.

With this change, we just print diffs in the simple unified format by
executing `diff -u`.

Fixes #342
linuxerwang/rules_go,linuxerwang/rules_go,Xjs/rules_go,linuxerwang/rules_go,linuxerwang/rules_go,bazelbuild/rules_go,Xjs/rules_go,linuxerwang/rules_go,Xjs/rules_go,bazelbuild/rules_go,bazelbuild/rules_go,Xjs/rules_go,bazelbuild/rules_go,Xjs/rules_go,bazelbuild/rules_go