test: catch squash author comparison issue

There is a bug in squash where author comparison uses the entire author
signature, including the author time, to determine whether all the
commits being squashed are by the same author.

The test case for this behavior creates patches fast enough that they
all share the same author time. Add artifical author times to the test
patches to trigger the bug.

Author: jpgrayson

t/t2600-squash.sh

@@ -73,11 +73,11 @@ test_expect_success 'Squash at stack top' '
 
 test_expect_success 'Squash patches with all non-default author' '
     echo "a" >>baz.txt &&
-    stg new -rm "a-patch" --author "Other Contributor <[email protected]>" &&
+    stg new -rm "a-patch" --author "Other Contributor <[email protected]>" --authdate 1762750000 &&
     echo "b" >>baz.txt &&
-    stg new -rm "b-patch" --author "Other Contributor <[email protected]>" &&
+    stg new -rm "b-patch" --author "Other Contributor <[email protected]>" --authdate 1762760000 &&
     echo "c" >>baz.txt &&
-    stg new -rm "c-patch" --author "Other Contributor <[email protected]>" &&
+    stg new -rm "c-patch" --author "Other Contributor <[email protected]>" --authdate 1762770000 &&
     stg squash -m "abc-patch" a-patch b-patch c-patch &&
     test_when_finished "stg delete abc-patch" &&
     stg show abc-patch | grep "Author:" >out &&