git projects with --separate-git-dir don't get processed by git-diff

[karldw]

Prerequisites

• Put an X between the brackets on this line if you have done all of the following:
  • Reproduced the problem in Safe Mode: http://flight-manual.atom.io/hacking-atom/sections/debugging/#using-safe-mode
  • Followed all applicable steps in the debugging guide: http://flight-manual.atom.io/hacking-atom/sections/debugging/
  • Checked the FAQs on the message board for common solutions: https://discuss.atom.io/c/faq
  • Checked that your issue isn't already filed: https://github.com/issues?utf8=✓&q=is%3Aissue+user%3Aatom
  • Checked that there is not already an Atom package that provides the described functionality: https://atom.io/packages

Description

When a git directory is initialized with --separate-git-dir (as an option to git init or git clone), git creates a .git file pointing to the repository location. It would be a nice feature if git-diff followed that pointer to perform diffs with the actual git repo.

Steps to Reproduce

1. Initialize a git repository with a separate repo directory:
   git clone --separate-git-dir=$HOME/separated_git-diff git@github.com:atom/git-diff.git git-diff1
2. Repeat without separate-git-dir:
   git clone git@github.com:atom/git-diff.git git-diff2
3. Open the projects in Atom:
   atom git-diff1 git-diff2

Expected behavior:
The two projects look and act the same.

Actual behavior:
The one with a separated git dir, git-diff1, is treated as a folder with no git repo.

In the screenshot below, I've added a line to the coffeelint file in both repos, but only git-diff2 has color.

Reproduces how often:
100% of the time.

Versions

Atom: 1.17.2
Electron: 1.3.15
Chrome: 52.0.2743.82
Node: 6.5.0

apm: 1.17.0
npm: 3.10.5
python 3.6.1
git: 2.13.0.windows.1
visual studio

Windows: 10

Additional Information

Related to atom/atom#2203
I used git bash for the command line steps, but the result is the same in git cmd.

[maxbrunsfeld]

Closing this as a duplicate of #134.