actions: Fix “Fix UserActivityInterval overlap bug” bug.
actions: Fix “Fix UserActivityInterval overlap bug” bug.

Commit 1a7ddd9ea3e5cb2784bba5cf1a51eec30ae756f4 “Fix
UserActivityInterval overlap bug” introduced a mathematically
incorrect assertion about how intervals work.  There’s a third way two
intervals could overlap: both the start and end of the old interval
could be inside the new interval.  This probably can’t happen here
because the old interval should be at least as long as the new
interval.  However, a correct overlap test can be formulated in a
simpler way anyway.

Signed-off-by: Anders Kaseorg <dfdb7392591db597bc41cf266a9c3bc12a2706e5@zulip.com>

rht/zulip,rht/zulip,rht/zulip,kou/zulip,zulip/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,punchagan/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,zulip/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,punchagan/zulip,kou/zulip,zulip/zulip,kou/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,andersk/zulip,hackerkid/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,kou/zulip,zulip/zulip