Merge branch 'kh/keep-tag-editmsg-upon-failure'

"git tag" learned to leave the "$GIT_DIR/TAG_EDITMSG" file when the command failed, so that the user can salvage what they typed. * kh/keep-tag-editmsg-upon-failure: tag: keep the message file in case ref transaction fails t/t7004-tag: add regression test for successful tag creation doc: tag: document `TAG_EDITMSG`