You never reach a phase when you can confidently say that you understand git. But it's certainly possible to go from "When something goes wrong, I just delete the repo and clone it again" to "Aha! Now I can deal with most of the issues".
Mine was when I realized that git commands come in two flavors. Those that deal with commits as snapshots (commit, checkout, switch, reset, etc) and those that deal with commits as changes/diffs/deltas (merge, rebase, cherrypick, revert, etc). (Note: This isn't about how git stores commits on disk). I believe that this is actually the main source of confusion for beginner and intermediate git users.
Still haven't. And I really dislike articles that say stuff like "git doesn't really store diffs!" followed by a bunch of paragraphs showing they don't understand what git does store or they are bad at explaining things (or both).
When I learned about the reflog. I became less afraid of my changes when I knew I could easily recover from my errors. This allowed me to experiment more with git and become more proficient in it.
Another aha moment was learning that an easy way to squash commits is just to do a git reset followed by git commit -am “whatever”
Every commit lists one or more parents, possibly several parents, like 8 parents. These commits thus form a graph structure.
Branches and labels are just references to commits in this graph structure; they are commit alias, just a name that references a specific commit. Branches and tags are the same, except by convention the CLI will move branches when you commit to a branch, but tags are not moved by the CLI.
(Commits may have many names, they have their commit ID, and they may also be named by a branch or tag. Commit IDs are hashes of the contents of the commit. This ensures, cryptographically, that a commit and it's ID can never change.)
Git never deletes a commit that is less than 90 days old. If you commit something, rest assured your work is in there somewhere, it's just that no mortal being may be able to find it. Deleting a branch removes a reference to a commit, but the commits in the branch are still there. The GUI tools usually hide commits that are not part of a branch, but you can see them using "reflog" related commands.
Fun fact. Such merges with more than 2 parents are called 'octopus merges'. The Linux repo has a single merge with 66 parents that Torvalds named the 'Cthulhu merge'.
Git never deletes a commit that is less than 90 days old.
On its own, that is. Not if you do a git gc.
Deleting a branch removes a reference to a commit, but the commits in the branch are still there.
but you can see them using “reflog” related commands
Reflog - one of the most underrated git commands that has the potential to save your life some day. At least one team member must learn it.
I discovered it, along with other DVCS when it came up, and looked into it and learned it. It was reasonable and intuitive enough for me. As far as I can remember anyway. (I don't have particular memories of that.)
That time I accidentally wiped an entire open source project on github and had to learn real quick how to undo a destructive commit.
Somehow after an entire night of google-fu, reading the git book three times, and tutorial videos, I got the right series of commands to fix it and nobody ever figured out what I did.
All I wanted to do was fix a typo in an imported module...
I initially just used it on personal projects just so I could rollback if I needed to. Afterwards I realized that you could branch after watching Fireship videos. I never got an aha moment, but the moment I really understood was after my first pull request to a project I liked at the time.