Incorrect background color when first line of diff hunk is added line

[dandavison]

When the first line of a diff hunk is an added line, the green background color of Magit incorrectly extends back into Magit's hunk header line, and one character of green remains visible when the hunk header is folded.

An example of the type of diff output that triggers this is

diff --git a/lib_fzf.sh b/lib_fzf.sh
index 0a16e24..6aef3c1 100644
--- a/lib_fzf.sh
+++ b/lib_fzf.sh
@@ -1,3 +1,8 @@
+fzf-git-checkout () {
+    git checkout $(git branch | fzf)
+}
+
+
 -fzf-hist () {
     dan-history | fzf --no-sort --exact
 }