Merge branch 'jk/maint-1.6.1-cleanup-after-exec-failure' into maint-1.6.1

* jk/maint-1.6.1-cleanup-after-exec-failure:
  git: use run_command() to execute dashed externals
  run_command(): help callers distinguish errors
  run_command(): handle missing command errors more gracefully
  git: s/run_command/run_builtin/