chore: Update for chrono deprecations

The non-deprecated east_opt() and timestamp_opt() interfaces return
Option/Result, but these changes stick with the expect/unwrap approach to
those outcomes. I.e. this change maintains StGit's status quo which is to
panic if a timestamp found in the git repo has an out-of-bounds timezone or
unix timestamp.

Author: jpgrayson

src/signature.rs

@@ -187,7 +187,10 @@ pub(crate) trait TimeExtended {
 
 impl TimeExtended for git2::Time {
     fn datetime(&self) -> DateTime<FixedOffset> {
-        FixedOffset::east(self.offset_minutes() * 60).timestamp(self.seconds(), 0)
+        FixedOffset::east_opt(self.offset_minutes() * 60)
+            .expect("tz offset minues is in bounds")
+            .timestamp_opt(self.seconds(), 0)
+            .unwrap()
     }
 
     fn epoch_time_string(&self) -> String {