Process MergeOutput to handle successful, failed, and halted merges

[nelsonni]

The results of merging can have varied outcomes that need to be properly handled, including:

• Fast Forward Merges
• 3-way Merges
• Rebasing
• Merge Conflicts
• Already up to date
• And a myriad of fatal and error messages indicating that the merge failed to start

To properly model these scenarios, this PR introduces processMergeOutput function for evaluating the raw output from an executed merge in order to discern status: Passing and status: Failing as well as reasons for these results.

This PR resolves #978.

Changes:

This PR makes the following changes:

• Rename TimelineButtons component to MergeButtons to reflect usage being dependent on merging
• Add git-rev-list command for listing commit objects in reverse chronological order
• Add processMergeOutput function for populating status, alreadyMerged, fastForward, output, mergeCommit, mergeStrategy, and conflicts fields from a merge
• Add test specs for processMergeOutput
• Add MergeStatus component for displaying success, warning, and error alert messages based on merge results
• Add React Developer Tools hook to Electron extensions