gitk: Remember time zones from author and commit timestamps

gitk: Remember time zones from author and commit timestamps

When resolving a conflicted cherry-pick, this lets us pass
GIT_AUTHOR_DATE to git citool with the correct timezone.
It does this by making elements 2 and 4 of the commitinfo array
entries, which store the author and committer dates of the commit,
be 2-element lists storing the numerical date and timezone offset,
rather than just the numerical date.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Paul Mackerras <paulus@samba.org>