Merge branch 'dl/format-patch-doc-test-cleanup'

The documentation and tests for "git format-patch" have been
cleaned up.

* dl/format-patch-doc-test-cleanup:
  config/format.txt: specify default value of format.coverLetter
  Doc: add more detail for git-format-patch
  t4014: stop losing return codes of git commands
  t4014: remove confusing pipe in check_threading()
  t4014: use test_line_count() where possible
  t4014: let sed open its own files
  t4014: drop redirections to /dev/null
  t4014: use indentable here-docs
  t4014: remove spaces after redirect operators
  t4014: use sq for test case names
  t4014: move closing sq onto its own line
  t4014: s/expected/expect/
  t4014: drop unnecessary blank lines from test cases