Truncate the beggining of the commands' output

Instead trunctaing the last part of the command, we truncate the beggining of
it. This allows to understand what's happening if the command fails (e.g. LaTeX
step) with more context to be able to solve it.

Author: humitos

readthedocs/builds/tasks.py

@@ -148,9 +148,9 @@ def archive_builds_task(days=14, limit=200, include_cold=False, delete=False):
         if data:
             for cmd in data:
                 if len(cmd['output']) > MAX_BUILD_COMMAND_SIZE:
-                    cmd['output'] = cmd['output'][:MAX_BUILD_COMMAND_SIZE]
-                    cmd['output'] += "\n\nCommand output too long. Truncated at 1MB."
-                    log.warning('Truncating build command for build %s', build.pk)
+                    cmd['output'] = cmd['output'][-MAX_BUILD_COMMAND_SIZE:]
+                    cmd['output'] = "... (truncated) ...\n\nCommand output too long. Truncated to last 1MB.\n\n" + cmd['output']  # noqa
+                    log.warning('Truncating build command for build. build=%s', build.pk)
             output = BytesIO()
             output.write(json.dumps(data).encode('utf8'))
             output.seek(0)