Merge branch 'jk/two-way-merge-corner-case-fix' into maint

"git am --abort" sometimes complained about not being able to write a tree with an 0{40} object in it. * jk/two-way-merge-corner-case-fix: t1005: add test for "read-tree --reset -u A B" t1005: reindent unpack-trees: fix "read-tree -u --reset A B" with conflicted index