Merge branch 'jk/external-diff-use-argv-array'

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