Squashing commits with git rebase -i may miscount commits in commit message

[Brandon Tolsch <btolsch@gmail.com>]

git --version: 2.11.0

When using git rebase -i to squash a series of commits that includes
more than 10 commits, the generated commit message you are given to
edit counts the old messages incorrectly.  It will say the total
number of commits is (actual % 10) (if they were 0-based) and it will
also count the commits as (actual % 10).

For example, 15, 25, 35, etc. commits:
# This is a combination of 5 commits.
# This is the 1st commit message:
msg

# This is the commit message #2:
...
...
# This is commit message #10:
msg

# This is commit message #1:
...

# This is commit message #5:
msg

While not a big issue, it did make me double check what I was doing
when I saw "a combination of 10 commits" instead of 20 in the commit
message.

-Brandon Tolsch