Include merge commits in the file history

[emanuel-v-r]

Currently when we use file history, it doesn't show any merge commits, which could be misleading, as it might contain changes for this specific file. Also, the popular git providers show merge commits when showing the history of a single file. Looks like the changes in this PR could fix it (currenlty it simply skips when there is more than one parent). Eventually if we want to keep the old behavior, we may want to pass some kind of flag to the FileHistory class, maybe includeMergeCommits=false