Sort seems to be unstable

[Spindletop]

Calling the sort of an already sorted file leads to wrong output.

This can be tested e.g. by calling sortxml.exe c_sorted.xml or
sortxml.exe file1.xml > file2.xml && sortxml.exe file2.xml > file3.xml && sortxml.exe file3.xml > file4.xml. file1.xml and file3.xml match, and file2.xml and file4.xml match, i.e. output toggles.

input file (c_sorted.xml)

<?xml version="1.0" encoding="UTF-8"?>
<CodeSnippets xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
  <CodeSnippet Format="1.0.0">
    <Header>
      <Author>Kody Brown</Author>
      <Description>Outputs: return false</Description>
      <HelpUrl>
      </HelpUrl>
      <Shortcut>rf</Shortcut>
      <SnippetTypes>
        <SnippetType>Expansion</SnippetType>
      </SnippetTypes>
      <Title>return false</Title>
    </Header>
    <Snippet>
      <Code Language="csharp"><![CDATA[return false;$end$]]></Code>
    </Snippet>
  </CodeSnippet>
  <CodeSnippet Format="1.0.0">
    <Header>
      <Author>Kody Brown</Author>
      <Description>Outputs: Loc.GetText("")</Description>
      <HelpUrl>
      </HelpUrl>
      <Shortcut>ll</Shortcut>
      <SnippetTypes>
        <SnippetType>SurroundsWith</SnippetType>
      </SnippetTypes>
      <Title>locgettext</Title>
    </Header>
    <Snippet>
      <Code Language="csharp"><![CDATA[Loc.GetText("$end$")]]></Code>
    </Snippet>
  </CodeSnippet>
</CodeSnippets>

output file:

<?xml version="1.0" encoding="ibm850"?>
<CodeSnippets xmlns="http://schemas.microsoft.com/VisualStudio/2005/CodeSnippet">
  <CodeSnippet Format="1.0.0">
    <Header>
      <Author>Kody Brown</Author>
      <Description>Outputs: Loc.GetText("")</Description>
      <HelpUrl>
      </HelpUrl>
      <Shortcut>ll</Shortcut>
      <SnippetTypes>
        <SnippetType>SurroundsWith</SnippetType>
      </SnippetTypes>
      <Title>locgettext</Title>
    </Header>
    <Snippet>
      <Code Language="csharp"><![CDATA[Loc.GetText("$end$")]]></Code>
    </Snippet>
  </CodeSnippet>
  <CodeSnippet Format="1.0.0">
    <Header>
      <Author>Kody Brown</Author>
      <Description>Outputs: return false</Description>
      <HelpUrl>
      </HelpUrl>
      <Shortcut>rf</Shortcut>
      <SnippetTypes>
        <SnippetType>Expansion</SnippetType>
      </SnippetTypes>
      <Title>return false</Title>
    </Header>
    <Snippet>
      <Code Language="csharp"><![CDATA[return false;$end$]]></Code>
    </Snippet>
  </CodeSnippet>
</CodeSnippets>

Program has been compiled with VS2012.