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/