Attempt to extract date from only the last line of git log output

Previously, the code assumes the output of `git log --format="%at"` only
contains commit timestamp. But this is not true.

`--format` only controls a portion of the output. `git log` command can
output additional information thats not controlled by format. e.g.: When
`showSignature` is set, it will always display commit signature info.

This commit attempts to locate timestamps only from the very last line
of `git log` output.

mkdocs_rss_plugin/util.py

@@ -149,14 +149,14 @@ def get_file_dates(
                         date="short",
                         format="%at",
                         diff_filter="AR",
-                    )
+                    ).split("\n")[-1]
                 if not dt_updated:
                     dt_updated = self.repo.log(
                         in_page.file.abs_src_path,
                         n=1,
                         date="short",
                         format="%at",
-                    )
+                    ).split("\n")[-1]
             except GitCommandError as err:
                 logging.warning(
                     "[rss-plugin] Unable to read git logs of '%s'. Is git log readable?"