[BUG?] 'git describe seen'?

[Junio C Hamano <gitster@pobox.com>]

This does not seem a new problem at all, as v2.10.0 thru more recent
versions of "git describe" seem to give me the same answer.

Anyway, I am seeing a curious symptom.

$ git rev-list --count v2.31.0..seen
716
$ git rev-list --count v2.31.1..seen
687

The above means that 'seen' is closer to v2.31.1 than v2.31.0; there
are fewer commits that are not in v2.31.1 that are in 'seen', than
commits that are not in v2.31.0 that are in 'seen'.  

That is naturally expected.

$ git rev-list --count v2.31.0..v2.31.1
29

And that difference of 29 matches the difference, which is 716 - 687.

But here is what is puzzling.

$ git describe seen
v2.31.0-716-g7b65b53281

$ git rev-list --first-parent master..seen |
  while read v
  do
	d=$(git describe $v)
	echo $v $d
	case "$d" in v2.31.1-*) break ;; esac
  done
7b65b53281ae06ee25dd47dead4062125eb54427 v2.31.0-716-g7b65b53281
eec14f0fec886c909a29d63a94537df5a62be618 v2.31.0-714-geec14f0fec
...
103835562c64abef2319995716230f92092f87af v2.31.0-569-g103835562c
d4324831d9152b16e091646e22a6e03423a59c93 v2.31.1-516-gd4324831d9

Is there something tricky about the topic merged at 10383556 (Merge
branch 'jh/rfc-builtin-fsmonitor' into seen, 2021-04-17) to confuse
the counting done in "git describe"?

$ git log --first-parent --oneline master..103835562^2
14d50074ff t7527: test status with untracked-cache and fsmonitor--daemon
07dbff70ce p7519: add fsmonitor--daemon
436807f77a t7527: create test for fsmonitor--daemon
c826602412 fsmonitor: force update index when fsmonitor token advances
aaaf17ce60 fsmonitor--daemon: use a cookie file to sync with file system
71a0e07d79 fsmonitor--daemon:: introduce client delay for testing
9aedb0f1ea fsmonitor--daemon: periodically truncate list of modified files
d4ae16b416 fsmonitor--daemon: implement handle_client callback
94f826fd48 fsmonitor-fs-listen-macos: implement FSEvent listener on MacOS
25663103ea fsmonitor-fs-listen-macos: add macos header files for FSEvent
e504205db7 fsmonitor-fs-listen-win32: implement FSMonitor backend on Windows
5e207d2af6 fsmonitor--daemon: create token-based changed path cache
42f493472f fsmonitor--daemon: define token-ids
082b8085d3 fsmonitor--daemon: add pathname classification
d4f4c59e56 fsmonitor--daemon: implement daemon command options
aec39650b5 fsmonitor-fs-listen-macos: stub in backend for MacOS
100cb9e8ad fsmonitor-fs-listen-win32: stub in backend for Windows
404d7dbdb6 fsmonitor--daemon: implement client command options
1f2d717bd6 fsmonitor--daemon: add a built-in fsmonitor daemon
4b10913cfe fsmonitor: introduce `core.useBuiltinFSMonitor` to call the daemon via IPC
867611645c config: FSMonitor is repository-specific
6e74370ce4 fsmonitor-ipc: create client routines for git-fsmonitor--daemon
ff338b5790 Merge branch 'jh/simple-ipc' into jh/rfc-builtin-fsmonitor