Minor improvements to git_changelog.

Minor improvements to git_changelog.

Avoid depending on Date::Calc, which isn't in a basic Perl installation,
when we can equally well use Time::Local which is.  Also fix the parsing
of timestamps to take heed of the timezone.  (It looks like cvs2git emitted
all commit timestamps with zone GMT, so this refinement might've looked
unnecessary when looking at converted data; but it's needed now.)

Fix parsing of message bodies so that blank lines that may or may not get
emitted by "git log" aren't confused with real data.  This avoids strange
formatting of the oldest commit on a branch.

Check child-process exit status, so that we actually notice if "git log"
fails, and so that we don't accumulate zombie children.