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

* jk/maint-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/

diff --cc git.c
Simple merge

diff --cc run-command.c
Simple merge

diff --cc run-command.h
Simple merge