completion: support git range-diff

Tab completion of `git range-diff` is very convenient, especially
given that the revision arguments to specify the commit ranges to
compare are typically more complex than, say, your grandfather's `git
log` arguments.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

squash! WIP completion: support `git range-diff`

Revert "WIP completion: support `git range-diff`"

This reverts commit 2e7af652af9e53a19fd947f8ebe37a78043afa49.

contrib/completion/git-completion.bash

@@ -1976,6 +1976,20 @@ _git_push ()
 	__git_complete_remote_or_refspec
 }
 
+_git_range_diff ()
+{
+  case "$cur" in
+  --*)
+          __gitcomp "
+	  	--creation-factor= --dual-color
+                  $__git_diff_common_options
+                  "
+          return
+          ;;
+  esac
+  __git_complete_revlist
+}
+
 _git_rebase ()
 {
 	__git_find_repo_path