Improve update_siggen.py to include authors

This improves the update_siggen.py to include the authors of commits at
the end of the commit summary line. We should be giving credit where
credit is due. This information shows up in the merge commits in siggen
as well as the HISTORY file and tag annotation.

Before:

```
Dest sha:   2edf215
Source sha: bd98591
fd9a519  Bug 1907894 - Add mozilla::Atomic<T> to the irrelevant signature list.
f51eb33  Bug 1906667 - Add mozilla::media::Interval<T> to the prefix list. (#6663)
e1d2b1a  Bug 1905714 - Only match JS::Call exactly in signatures. (#6652)
8cf6cf3  bug-1904528: add SafeVariantClear and VariantCopy to prefix list
```

After:

```
Dest sha:   2edf215
Source sha: bd98591
fd9a519  Bug 1907894 - Add mozilla::Atomic<T> to the irrelevant signature list.  (Andrew McCreight)
f51eb33  Bug 1906667 - Add mozilla::media::Interval<T> to the prefix list. (#6663)  (Andrew McCreight)
e1d2b1a  Bug 1905714 - Only match JS::Call exactly in signatures. (#6652)  (Andrew McCreight)
8cf6cf3  bug-1904528: add SafeVariantClear and VariantCopy to prefix list  (Will Kahn-Greene)
```

bin/update_siggen.py

@@ -60,7 +60,7 @@ def main(argv=None):
                 "git",
                 "log",
                 "%s..%s" % (dest_sha, source_sha),
-                "--oneline",
+                "--pretty=%h  %s  (%an)",
                 "socorro/signature",
             ]
         )