Merge branch 'jk/stash-require-clean-index' into maint

"git stash pop/apply" forgot to make sure that not just the working tree is clean but also the index is clean. The latter is important as a stash application can conflict and the index will be used for conflict resolution. * jk/stash-require-clean-index: stash: require a clean index to apply t3903: avoid applying onto dirty index t3903: stop hard-coding commit sha1s