On 20 Feb 2018, at 3:39, Evan Jones wrote:

Something's gone wonky with my Javascript bundle (and only that one, as far as I can tel)l. Every time I hit cmd-/ to comment/uncomment some text, it uses an unindented block comment instead of the double-slash single line comment I expect. ("/* */", rather than "// ")
[…]
A) how I could fix this particular issue, or
B) how I could remove all my changes to the Javascript bundle and revert simply to the repo version?

In a JavaScript file select Bundles → Select Bundle Item… (⌃⌘T) and switch to “Settings” then enter comment, you should see something like this:

From this window, you can edit the settings, but any changes you have made to the JavaScript (and other bundles) should be located in ~/Library/Application Support/TextMate. So a rough way to revert to defaults is simply to remove that folder (or the bundles contained in it).

I would still suggest first looking at the settings via the above dialog, because the issue might also be that you installed a third party bundle that override the comment settings for the JavaScript scope.