🐛 Teach git-repository-provider to recognize .git-files

Git repositories may be contained in a .git directory or a .git file
in the workdir hierarchy, but Atom only recognizes the directory format.
Teach Atom to recognize the filesystem-agnostic Git symbolic link
used by default in many situations including, for example, submodules.

The .git file contains a relative or absolute path to the location of
the real git-dir, preceded by the 8-byte string "gitdir: ".

Here's a console log showing the normal creation of such a symbolic link.

    /tmp $ git init --separate-git-dir foo.git bar
    Initialized empty Git repository in /tmp/foo.git/
    /tmp $ ls
    /tmp $ bar  foo.git
    /tmp $ ls -la bar
    drwxr-xr-x 2 hordp hordp 4096 Sep 18 15:54 .
    drwxr-xr-x 4 hordp hordp 4096 Sep 18 15:54 ..
    -rw-r--r-- 1 hordp hordp   25 Sep 18 15:54 .git
    /tmp $ ls foo.git
    branches  config  description  HEAD  hooks  info  objects  refs
    /tmp $ cat bar/.git
    gitdir: /tmp/foo.git

Fixes #8876

src/git-repository-provider.coffee

@@ -1,6 +1,17 @@
 fs = require 'fs'
+{Directory} = require 'pathwatcher'
 GitRepository = require './git-repository'
 
+# Returns the .gitdir path in the agnostic Git symlink .git file given, or
+# null if the path is not a valid gitfile.
+#
+# * `gitFile` {String} path of gitfile to parse
+gitFileRegex = RegExp "^gitdir: (.+)"
+pathFromGitFile = (gitFile) ->
+  try
+    gitFileBuff = fs.readFileSync(gitFile, 'utf8')
+    return gitFileBuff?.match(gitFileRegex)[1]
+
 # Checks whether a valid `.git` directory is contained within the given
 # directory or one of its ancestors. If so, a Directory that corresponds to the
 # `.git` folder will be returned. Otherwise, returns `null`.
@@ -11,6 +22,9 @@ findGitDirectorySync = (directory) ->
   # can return cached values rather than always returning new objects:
   # getParent(), getFile(), getSubdirectory().
   gitDir = directory.getSubdirectory('.git')
+  gitDirPath = pathFromGitFile(gitDir.getPath?())
+  if gitDirPath
+    gitDir = new Directory(directory.resolve(gitDirPath))
   if gitDir.existsSync?() and isValidGitDirectorySync gitDir
     gitDir
   else if directory.isRoot()