update a.txt
Revert "update a.txt"

This reverts commit c3fe2322c350f6d4c5354945ff685b891706caf5.
update a.txt
first-time commit
second-time commit
first-time commit in B1
second-time commit in B1
third-time commit in B1
fourth-time commit in B1
fifth-time commit in B1
sixth-time
merge B1 into master
update a
merge B1 into master
git pull master