upload: Add --trim-tags=nonidentifying

[jerry-skydio]

Trimming every tag makes the pushed commits unusable for making the same
pr again, since a cherry-pick of one no longer says which topic, branch
or relative topic it belongs to. Trimming nothing leaves lines that stop
mattering the moment the pr exists, since the forge remembers labels,
reviewers and assignees once they've been set.

Add a middle setting that only trims those, plus update-pr-body and any
tag left with no value. --trim-tags now takes false, nonidentifying or
true, and accepts the same values from config as before. The value is
mandatory so that the flag never reads like a boolean, which would
invite writing a topic name where the value goes.

Tag line matching moves out of parse_commit_tags into match_commit_tag
so the new trim can reuse the case and plural handling.

[jerry-skydio]

Reviews in this chain:
└#296 upload: Add --trim-tags=nonidentifying

[jerry-skydio]

#	head	base	diff	date	summary
0	0e5c3632	8e33338d	diff	Sep 11 14:51 PM	6 files changed, 149 insertions(+), 39 deletions(-)
1	be994817	8e33338d	diff	Sep 11 14:59 PM	3 files changed, 12 insertions(+), 4 deletions(-)
2	54512aef	09ceb4dc	diff	Sep 15 13:20 PM	0 files changed
3	285a1d7a	fb24bd7e	diff	Sep 15 14:05 PM	1 file changed, 2 insertions(+), 3 deletions(-)
4	e9fd5a7d	fb24bd7e	diff	Sep 15 14:07 PM	1 file changed, 2 insertions(+), 5 deletions(-)