[it] updating chapter6

Reviewed "A SHORT NOTE ABOUT SHA-1"
update `git reflog` output to current one
[it] fix error in formating
[it] fix remaining typos
[it] fix space at end of title
switch travis to parallel ci check
travis: add e-mail notification on status change
Apply 60e2618 to ja tree
Apply c418e2e to ja tree
Apply 44936fd to ja tree
Apply 3f6c105 to ja tree
[nl] Synchronized translation with SHA-1 3c18779
[it] reviewing chapter 6

reviewed "Branch References"
[it] reviewing chapter 6

reviewed "RefLog Shortnames"
[it] reviewing chapter 6

reviewed "Ancestry References"
[it] reviewing chapter 6

Reviewed "Commit Ranges"
[it] reviewing chapter 6
[it] reviewing chapter 6

Reviewed "Double Dot"
[it] reviewing chapter 6

Multiple Points
[it] reviewing chapter 6

Triple Dot