Jump no more than a month at a time

This prevents breaking when jumping past early 2017, by which point
we're jumping too fast and run into our early boundary.

There could be further polish here, but this seems like an easy and
reasonable fix for the time being.

Author: Mark-Simulacrum

src/main.rs

@@ -798,7 +798,9 @@ fn bisect_nightlies(cfg: &Config, client: &Client) -> Result<BisectionResult, Er
                     last_failure = nightly_date;
                 }
                 nightly_date = nightly_date - chrono::Duration::days(jump_length);
-                jump_length *= 2;
+                if jump_length < 30 {
+                    jump_length *= 2;
+                }
                 if !cfg.args.preserve {
                     let _ = t.remove(&dl_spec);
                 }