remote-hg: avoid buggy strftime()

  error on pull: fatal: Invalid raw date "" in ident: remote-hg <>

Neither %s nor %z are officially supported by python, they may work on
some (most?) platforms, but not all.

removed strftime use of %s and %z, which are not officially supported by python, with standard formats

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
remote-helpers: add extra safety checks

Suggested-by: Roman Ovchinnikov <coolthecold@gmail.com>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>