Don't add 'topcrash' keyword back, if a human has removed it and there's another bug with the same signature and the 'topcrash' keyword

(Context: https://bugzilla.mozilla.org/show_bug.cgi?id=2007715#c7 and following failed attempts to remove the topcrash signature)

Sometimes we have multiple distinct bugs that happen to share the same crash-signature. If one of them happens to be a topcrash (making the shared crash-signature show up as a topcrash), then bugbot adds the topcrash signature to all bugs with that signature.[1]

This makes it look (on e.g. dashboards like https://bugdash.moz.tools/ ) like we have multiple distinct topcrashes, when in fact we only have one bug that's a topcrash (and some others that happen to share the same signature but have much lower crash volume).

After making this determination, a human engineer (e.g. me in https://bugzilla.mozilla.org/show_bug.cgi?id=2007715#c7 ) may decide to remove the topcrash signature from the low-volume bugs, but bugbot will just add it back (modulo [1]).

After discussing this situation with @suhaibmujahid , we think maybe it'd make sense for the bot to not add the keyword back in this situation, IF there's another bug open that shares the same crash signature, and that bug has the topcrash keyword (so the bot can be confident that the topcrash is still tracked somewhere).

(Not sure if this is common enough to bother adding this logic, but filing this issue as a reminder to look into it if this comes up again.)

[1] (one exception where the bot doesn't add the topcrash keyword is where the bot itself has previously removed it from that bug, e.g. for crashes that drift onto & off of & back onto the topcrash list)

Start by locating the bugbot logic that adds the topcrash keyword and review how it handles shared crash signatures. Reproduce the case described with multiple open bugs sharing a signature, one retaining topcrash and one having it removed. Done means the bot does not restore the keyword to the latter when another matching bug still tracks it.