Merge conflicts do not detect diff3 conflict style

[gediminasz]

• VSCode Version: 1.13.0
• OS Version: macOS Sierra 10.12.5

Steps to Reproduce:

1. Set git conflict style to diff3. In ~/.gitconfig:

[merge]
    conflictstyle = diff3

2. This results in merge conflict format as shown below. You can copy paste this into a blank tab:

<<<<<<< Updated upstream
foo
||||||| merged common ancestors
bar
=======
baz
>>>>>>> Stashed changes

3. Screenshot:

4. If you click "Accept Current Change", the "merged common ancestors" remains:

Some other notes:

• "Accept Incoming Change" works as expected.
• "Accept Both Changes" results in the following:

• "Compare Changes" fails with error if not in git repository, or something like this:

(Note the trailing "merged common ancestors" on the left side)

[chrmarti]

#27405 by @katemihalikova will take care of this.