Improve "git p4" on Cygwin.

* pw/git-p4-on-cygwin: (21 commits)
  git p4: introduce gitConfigBool
  git p4: avoid shell when calling git config
  git p4: avoid shell when invoking git config --get-all
  git p4: avoid shell when invoking git rev-list
  git p4: avoid shell when mapping users
  git p4: disable read-only attribute before deleting
  git p4 test: use test_chmod for cygwin
  git p4: cygwin p4 client does not mark read-only
  git p4 test: avoid wildcard * in windows
  git p4 test: use LineEnd unix in windows tests too
  git p4 test: newline handling
  git p4: scrub crlf for utf16 files on windows
  git p4: remove unreachable windows \r\n conversion code
  git p4 test: translate windows paths for cygwin
  git p4 test: start p4d inside its db dir
  git p4 test: use client_view in t9806
  git p4 test: avoid loop in client_view
  git p4 test: use client_view to build the initial client
  git p4: generate better error message for bad depot path
  git p4: remove unused imports
  ...