Shamelessly yank the merge-conflict parser and an initial UI take based on merge-conflicts to resolve merge conflicts.

• Parse conflicting hunks in TextEditors that are open on files that currently include merge conflicts. Handle all of those weird recursive diff3 output shenanigans.
• Add decorations within the TextEditor to identify the ours, theirs, and potentially parent sections. Style the lines within each conflict hunk.
• Include controls for simple resolutions within the conflict hunk decoration.
• Include a gear icon that triggers a dropdown menu offering more complex resolutions (ours-then-theirs, theirs-then-ours, and so on). See Add a "Keep Both" strategy? smashwilson/merge-conflicts#185.
• Add a progress bar to each entry in the conflicting files list to indicate the number of conflict hunks that still need to be resolved within each file.
• Warn when staging conflicting files that contain unresolved hunks.

/cc @iolsen, @BinaryMuse, @kuychaco