Add a sanity check on the output

Check that no line from any of the input files was lost.

This is not perfect for several reasons.
It doesn't check that the content goes to the desired location.
It doesn't check that sections are created as necessary.
It doesn't support whitespace normalization that the parsing code does.
But it's a good start.

scripts/assemble_changelog.py

@@ -32,6 +32,11 @@ class InputFormatError(Exception):
                                      message.format(*args, **kwargs))
         super().__init__(message)
 
+class LostContent(Exception):
+    def __init__(self, filename, line):
+        message = ('Lost content from {}: "{}"'.format(filename, line))
+        super().__init__(message)
+
 STANDARD_SECTIONS = (
     b'Interface changes',
     b'Default behavior changes',
@@ -179,9 +184,30 @@ class ChangeLog:
             for line in self.trailer:
                 out.write(line)
 
-def finish_output(changelog, output_file):
+def check_output(generated_output_file, main_input_file, merged_files):
+    """Make sanity checks on the generated output.
+
+    The intent of these sanity checks is to have reasonable confidence
+    that no content has been lost.
+
+    The sanity check is that every line that is present in an input file
+    is also present in an output file. This is not perfect but good enough
+    for now.
+    """
+    generated_output = set(open(generated_output_file, 'rb'))
+    for line in open(main_input_file, 'rb'):
+        if line not in generated_output:
+            raise LostContent('original file', line)
+    for merged_file in merged_files:
+        for line in open(merged_file, 'rb'):
+            if line not in generated_output:
+                raise LostContent(merged_file, line)
+
+def finish_output(changelog, output_file, input_file, merged_files):
     """Write the changelog to the output file.
 
+    The input file and the list of merged files are used only for sanity
+    checks on the output.
     """
     if os.path.exists(output_file) and not os.path.isfile(output_file):
         # The output is a non-regular file (e.g. pipe). Write to it directly.
@@ -191,6 +217,7 @@ def finish_output(changelog, output_file):
         # then move it into place atomically.
         output_temp = output_file + '.tmp'
     changelog.write(output_temp)
+    check_output(output_temp, input_file, merged_files)
     if output_temp != output_file:
         os.rename(output_temp, output_file)
 
@@ -215,7 +242,7 @@ def merge_entries(options):
     for filename in files_to_merge:
         with open(filename, 'rb') as input_file:
             changelog.add_file(input_file)
-    finish_output(changelog, options.output)
+    finish_output(changelog, options.output, options.input, files_to_merge)
     if not options.keep_entries:
         remove_merged_entries(files_to_merge)