Diff fails with NoMethodError: undefined method '[]' for nil:NilClass when line ending warnings are emitted

[glennsarti]

From ruby-git/ruby-git#326

Repro case:

• Download a repository which contains a file with CRLF endings on an operating system that uses LF endings
• Extracting the diff will throw a NoMethodError: undefined method '[]' for nil:NilClass error in method process_full_diff

Example output;

Overriding any local changes to repositories in modules
Not managing Rakefile in puppetlabs-chocolatey
Not managing spec/spec_helper.rb in puppetlabs-chocolatey
Using no-op. Files in puppetlabs-chocolatey may be changed but will not be committed.
Files changed:
Error while updating puppetlabs-chocolatey
bundler: failed to load command: msync (/mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/bin/msync)
NoMethodError: undefined method `[]' for nil:NilClass
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/gems/git-1.3.0/lib/git/diff.rb:151:in `block in process_full_diff'
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/gems/git-1.3.0/lib/git/diff.rb:134:in `each'
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/gems/git-1.3.0/lib/git/diff.rb:134:in `process_full_diff'
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/gems/git-1.3.0/lib/git/diff.rb:109:in `process_full'
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/gems/git-1.3.0/lib/git/diff.rb:68:in `each'
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/bundler/gems/modulesync-899483f48304/lib/modulesync/git.rb:162:in `update_noop'
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/bundler/gems/modulesync-899483f48304/lib/modulesync.rb:114:in `manage_module'
  /mnt/c/Source/modulesync_configs/.bundle/bash/ruby/2.3.0/bundler/gems/modulesync-899483f48304/lib/modulesync.rb:133:in `block in update'

The reason it fails because the code at; https://github.com/schacon/ruby-git/blob/master/lib/git/diff.rb#L139-L151 expects the first line to contain the filename whereas raw diff comes back as;

warning: CRLF will be replaced by LF in examples/init.pp.
The file will have its original line endings in your working directory.
diff --git a/.gitattributes b/.gitattributes
index 900ea0c..02d4646 100644
--- a/.gitattributes
+++ b/.gitattributes
@@ -1,5 +1,5 @@
 #This file is generated by ModuleSync, do not edit.

---

This issue is track the monkey patch that is in #111 (comment)