Incorrect unified diff when run with "--find-copies-harder"

Hello,

I believe I found a bug in how Git represents a diff when invoked with "--find-copies-harder" parameter. Specifically, the unified diff header of a hunk contains an extra piece of text which appears to be a line from the context (i.e. unchanged line), something like this:

    > git diff --find-copies-harder d00ca3f 20fb313
    diff --git a/test.txt b/copy.txt
    similarity index 81%
    copy from test.txt
    copy to copy.txt
    index 734156d..43a3f9d 100644
    --- a/test.txt
    +++ b/copy.txt
    @@ -2,6 +2,7 @@ line 1
     line 2
     line 3
     line 4
    +added line
     line 5
     line 6
     line 7

Note "line 1" after the standard unified diff header.

I prepared a sample repository with a minimal file I can reproduce this problem with: https://bitbucket.org/dpenkin/find-copies-harder-bug

I'm running Git 2.18.0 on a macOS, but I also tried with Git 2.15.0 and 2.8.6 running on Alpine Linux and was able to reproduce the same problem.

Please advise whether this is expected output or is indeed a bug.

Thank you.

Kind regards, Daniil Penkin