Don't modify arguments in CommitRange#initialize

This method used to call strip! on input strings which will mess with
the strings if they're re-used or frozen.

app/models/commit_range.rb

@@ -62,7 +62,7 @@ class CommitRange
   def initialize(range_string, project)
     @project = project
 
-    range_string.strip!
+    range_string = range_string.strip
 
     unless range_string =~ /\A#{PATTERN}\z/
       raise ArgumentError, "invalid CommitRange string format: #{range_string}"

spec/models/commit_range_spec.rb

@@ -24,6 +24,16 @@ describe CommitRange, models: true do
     expect { described_class.new("Foo", project) }.to raise_error(ArgumentError)
   end
 
+  describe '#initialize' do
+    it 'does not modify strings in-place' do
+      input = "#{sha_from}...#{sha_to}   "
+
+      described_class.new(input, project)
+
+      expect(input).to eq("#{sha_from}...#{sha_to}   ")
+    end
+  end
+
   describe '#to_s' do
     it 'is correct for three-dot syntax' do
       expect(range.to_s).to eq "#{full_sha_from}...#{full_sha_to}"