get_maintainer: remove stray punctuation when cleaning file emails

get_maintainer: remove stray punctuation when cleaning file emails

When parsing emails from .yaml files in particular, stray punctuation
such as a leading '-' can end up in the name.  For example, consider a
common YAML section such as:

  maintainers:
    - devicetree@vger.kernel.org

This would previously be processed by get_maintainer.pl as:

  - <devicetree@vger.kernel.org>

Make the logic in clean_file_emails more robust by deleting any
sub-names which consist of common single punctuation marks before
proceeding to the best-effort name extraction logic.  The output is then
correct:

  devicetree@vger.kernel.org

Some additional comments are added to the function to make things
clearer to future readers.

Link: https://lore.kernel.org/all/0173e76a36b3a9b4e7f324dd3a36fd4a9757f302.camel@perches.com/
Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Alvin Šipraga <alsi@bang-olufsen.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>