style: add rule to ignore *.orig files in .gitignore

By git mergetool docs:
> git mergetool creates *.orig backup files while resolving merges. These
> are safe to remove once a file has been merged and its git mergetool
> session has completed.

These files don't serve any use case in the source version control, so
are going to be ignored from version control from this commit on.

Author: nymius

.gitignore

@@ -1,2 +1,3 @@
 target/
 Cargo.lock
+*.orig