Add support for submodules

[kuychaco]

This involves adding submodule support to what-the-diff

[BinaryMuse]

Hm, what did we mean by this, exactly? Currently this gets parsed as a regular text file and shows in the unstaged changes correctly:

~/src/node-cmark on master! $ git status
On branch master
Your branch is up-to-date with 'origin/master'.
Changes not staged for commit:
  (use "git add <file>..." to update what will be committed)
  (use "git checkout -- <file>..." to discard changes in working directory)

	modified:   README.md
	modified:   cmark (new commits)
	modified:   package.json

no changes added to commit (use "git add" and/or "git commit -a")

~/src/node-cmark on master! $ git diff -- cmark
diff --git a/cmark b/cmark
index bb8ada5..4e9fad6 160000
--- a/cmark
+++ b/cmark
@@ -1 +1 @@
-Subproject commit bb8ada50e5ca66aa5356d692c7bafa444f8460a5
+Subproject commit 4e9fad6a7183c7a1fe1fd631f2e36547631fafbc

👇

[BinaryMuse]

Maybe we should support showing changes down in a submodule eventually? But for now I think the above is fine to staff ship with, unless there's some edge case I'm missing.

[smashwilson]

From the linked issue in what-the-diff, it looks like it used to throw a stacktrace in the parser.

Also, does it break if git config --global diff.submodule log is set?

[maxbrunsfeld]

When my project folder is a submodule, I can view the diff, but I cannot stage or unstage hunks or files; is this a known issue?

[kevinansfield]

I have the following git structure:

ghost (primary git repo)
|-- core/client (submodule)
|-- content/themes/casper (submodule)

I mostly work in the client by running cd core/client then atom .. All the git operations performed via terminal in that directory treat the submodule as if it is a separate repository which is what I want as it is a completely separate project to the main repo.

However I've just tried the Atom Beta by running atom-beta . in the core/client directory and it's only picking up the top-level Ghost repository which isn't of any use because all the commits, PRs etc are on the submodule's repository.

Is this the same problem that this issue is referring to?

[BinaryMuse]

it's only picking up the top-level Ghost repository

Okay, @kuychaco and I just tracked this down (and learned something new about Git in the process. happens a lot on this project 😅)

So submodule working directories do contain a .git entry, but it's a file, not a folder, and the file contains some text that points to the actual repository directory, which lives in the containing repository's .git/modules directory. This is problematic for two reasons:

1. WorkdirCache#walkToRoot walks up the folder hierarchy trying to find a .git folder, and it fails if it finds one but it's not a file. This needs to be adjusted to account for the pseudo-symlink nature of the .git file in submodule working dirs.
2. We make assumptions throughout the codebase (probably mostly in Repository and friends and GitShellOutStrategy) that the .git folder is a subdirectory of the working directory (to read things like MERGE_HEAD, etc.) We should implement a smarter getGitDirectoryPath and use it everywhere instead.

---

I can't say for sure how long it'll be before we have full submodule support in Atom, but in the meantime we are trying to take care of some of the more egregious errors and problems (e.g. #828). I'll open some issues to track specific tasks. Thanks for your help tracking this all down! ✨