Code clean-up (and a bugfix which has been merged for 2.0).
* jk/external-diff-use-argv-array:
run_external_diff: refactor cmdline setup logic
run_external_diff: hoist common bits out of conditional
run_external_diff: drop fflush(NULL)
run_external_diff: clean up error handling
run_external_diff: use an argv_array for the environment