If so, I would describe the index as a sort of staging area for preparing your commit. You might not necessarily want to include every single change in your workspace in your next commit. The index allows you to pick which things you want to go into the commit then git-commit creates the commit from what's in the index. If you don't care for such behaviour, and you just want to commit all changes in tracked files in your workspace, git-commit -a does that.
This[0] is one of the best tools I've seen for understanding git commands and even a bit of how git works. It's interactive, divides things into the different 'places' that content can be in git and then shows you how each command moves content between those places. Click on the workspace and it will show each command which does things to content in your workspace, the bar the command is written on shows what the other area it interacts with is and the direction that the command moves content. I hope it helps. [0] http://ndpsoftware.com/git-cheatsheet.html
I might add that if you didn't have the index, then you could not make a commit that contained an add, a rename, and a deletion. If you think far too deeply about how Subversion handles these operations, it becomes clear that -conceptually- Subversion had an index/"staging area", too. Ferinstance, the output of 'svn help add' says
add: Put files and directories under version control, scheduling
them for addition to repository. They will be added in next commit.
usage: add PATH...
The big conceptual distinction between SVN and git is that with SVN you tie exactly one repository to a given working copy. In git you can tie multiple repos to a given working copy and (by default) one of those repos is stored in the same place as the working copy.
Does that make sense, or is the index still somewhat-to-rather unclear and/or mystifying? (I mean, other than its kinda crappy name.)
> ...can you point me at something not written by the git authors that explains what the hell is going on here and why?
If "here" is "with git in general", I read the Git Book [0] ages ago, and combined what I learned from it with a fair amount of fucking around with my repo, and also with the contents of the gittutorial(7), gittutorial-2(7) and (parts of) gitglossary(7) man pages. [1]
From looking at the ToC of the Git Book, it looks like chapters 1, 2, 3, and 7 would be relevant to your interest. Chapters 5 and 10 might be relevant. I can't offer any guarantees, as I last read the Git Book ages ago, and this look like it's a new version... the one I read didn't make any mention of Github.
Though, if you were asking about something more specific, I'm happy to take a stab at answering that question once I know what it is. :)
[0] https://git-scm.com/book/en/v2
[1] Even though you asked for things not written by the git guys, I got a fair bit of value from the official git tutorials. It's also possible that you've overlooked them, so I bring them up.
Perhaps my biggest point of confusion with git comes from that nebulous intermediate structure which sits between the real working directory and the real repository, which sort of acts like a repository and sort of acts like a working directory. It has many names and no clear purpose, and it doesn't fit into my mental model of the work to be done when working with a VCS.
Your explanation of merges makes more sense from that context. I don't think of add/mv/rm as operations on the nebulous repository, because I don't have any idea why one operates on the nebulous repository; what I'm trying to do is tell git to track a file, or stop tracking a file, or notice that I've moved a file from one place to another. The fact that these operations also kind of half-commit changes to this semi-repository is just... confusing, because I don't know why one would care.
If the pseudo-semi-repository thingy actually made sense, perhaps it would seem more natural that add/mv/rm do things to it during merges. I suspect the behavior of checkout, reset, and commit might also make more sense; as is, they seem to be needlessly complex, because I am never manipulating the semi-repository on purpose: I'm either trying to move my changes from the working directory into the local repository, or I'm trying to update my working directory to match the local repository, but in no case am I ever trying to half-update the intermediate state I can't actually see.
Given this somewhat confused explanation and the fact that you've done a great job of explaining what git is doing so far, can you point me at something not written by the git authors that explains what the hell is going on here and why? I would like to understand the tools I'm using instead of just blindly typing arcane rituals cribbed from the internet, but as I said before trying to read the git documentation just leaves me more confused than before I started.