Merge branch 'jk/index-pack-wo-repo-from-stdin' into maint

"git index-pack --stdin" needs an access to an existing repository, but "git index-pack file.pack" to generate an .idx file that corresponds to a packfile does not. * jk/index-pack-wo-repo-from-stdin: index-pack: skip collision check when not in repository t: use nongit() function where applicable index-pack: complain when --stdin is used outside of a repo t5000: extract nongit function to test-lib-functions.sh