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