Volker found this and Microsoft Dochelp confirmed this is a Windows bug. Fix from Volker to follow.
Created attachment 11536 [details]
git-am fix for 4.3.next, 4.2.next.
Cherry-pick from master.
Re-assigning to Karolin for inclusion in 4.3.next, 4.2.next.
Pushed to autobuild-v4-[3|2]-test.
(In reply to Karolin Seeger from comment #3)
Pushed to both branches.
Closing out bug report.