Introduce the ability to filter commits to only include merge commits when using Git::Base#log.

Example:

log = git.log
  .max_count(:all)
  .merges

This enhancement adds a new method to the Git::Log class to enable this functionality and updates the relevant log commands accordingly.

Since the --merges option to git log only filters the results, I thought it was sufficient to just test that the option was added to the git log command when appropriate.

Fixes #788