Merge branch 'sp/runtime-prefix'

Merge branch 'sp/runtime-prefix'

* sp/runtime-prefix:
  Windows: Revert to default paths and convert them by RUNTIME_PREFIX
  Compute prefix at runtime if RUNTIME_PREFIX is set
  Modify setup_path() to only add git_exec_path() to PATH
  Add calls to git_extract_argv0_path() in programs that call git_config_*
  git_extract_argv0_path(): Move check for valid argv0 from caller to callee
  Refactor git_set_argv0_path() to git_extract_argv0_path()
  Move computation of absolute paths from Makefile to runtime (in preparation for RUNTIME_PREFIX)