support THOR_MERGE values with arguments

- Split THOR_MERGE with Shellwords and pass argv to system
- Adds spec covering THOR_MERGE="nvim -d"

Fixes #909

Author: rafaelfranca

lib/thor/shell/basic.rb

@@ -372,12 +372,15 @@ def merge(destination, content) #:nodoc:
         Tempfile.open([File.basename(destination), File.extname(destination)], File.dirname(destination)) do |temp|
           temp.write content
           temp.rewind
-          system(merge_tool, temp.path, destination)
+          system(*merge_tool, temp.path, destination)
         end
       end
 
       def merge_tool #:nodoc:
-        @merge_tool ||= ENV["THOR_MERGE"] || "git difftool --no-index"
+        @merge_tool ||= begin
+          require "shellwords"
+          Shellwords.split(ENV["THOR_MERGE"] || "git difftool --no-index")
+        end
       end
     end
   end

spec/shell/basic_spec.rb

@@ -564,6 +564,13 @@ def  #456  Lanç...
         capture(:stdout) { shell.file_collision("foo") {} }
       end
 
+      it "invokes the merge tool with arguments when THOR_MERGE contains them" do
+        allow(ENV).to receive(:[]).with("THOR_MERGE").and_return("nvim -d")
+        expect(Thor::LineEditor).to receive(:readline).and_return("m")
+        expect(shell).to receive(:system).with("nvim", "-d", /foo/, "foo")
+        capture(:stdout) { shell.file_collision("foo") {} }
+      end
+
       it "show warning if user chooses merge but merge tool is not specified" do
         allow(shell).to receive(:merge_tool).and_return("")
         expect(Thor::LineEditor).to receive(:readline).and_return("m")