Issue 449 - update the Git json files

[raduiman-zz]

• This PR will implement the functionality to update the GIT .json files with the new commits , the last check date time and the last release version.
• The .json files have the following structure:
  • last check and latest version
  • the commits, starting with the newest at the top of .json

_get-release() - https://github.com/mozilla-releng/firefox-infra-changelog/commit/86704202c9a3397bcadf2e48690362b42c6933b4

This commit changes the method to return a release version of our choosing by accepting an integer argument. If the argument is "1" it will return the latest version, "2" will return the second to latest version, and so on

_generate_first_element() - https://github.com/mozilla-releng/firefox-infra-changelog/commit/72e30c75764df11e7f1f8e1567397bd47b8c5ee3

This method will generate the element 0 for the .json files. It will contain the last check date and the last official release.

_write_git_json() - https://github.com/mozilla-releng/firefox-infra-changelog/commit/72e30c75764df11e7f1f8e1567397bd47b8c5ee3

This method will update the .json files. It will fuse the first element, the new commit dictionary and the old commit dictionary all in one and call the save function to write the .json.

[zsoltfay]

The latest commit 1d1eaee brought the following changes:

• deleted the below code from line 139+140:

     else:
          self.local_version = None

• Moved self.commit_number = 0 into start_git()
• Moved self.list_of_commits.update(self._generate_first_element()) from _write_git_json into start_git()
• Re-wrote parts of _write_git_json to suite every scenario of writing to file(see the comment in the code)

Tested the code by running it with no files present locally and then re-running it multiple times. No errors