Merge branch 'js/var-git-shell-path'

"git var GIT_SHELL_PATH" should report the path to the shell used
to spawn external commands, but it didn't do so on Windows, which
has been corrected.

* js/var-git-shell-path:
  var(win32): do report the GIT_SHELL_PATH that is actually used
  run-command: declare the `git_shell_path()` function globally
  run-command(win32): resolve the path to the Unix shell early
  mingw(is_msys2_sh): handle forward slashes in the `sh.exe` path, too
  win32: override `fspathcmp()` with a directory separator-aware version
  strvec: declare the `strvec_push_nodup()` function globally
  run-command: refactor getting the Unix shell path into its own function