run_external_diff: use an argv_array for the command line

We currently generate the command-line for the external
command using a fixed-length array of size 10. But if there
is a rename, we actually need 11 elements (10 items, plus a
NULL), and end up writing a random NULL onto the stack.

Rather than bump the limit, let's just use an argv_array, which
makes this sort of error impossible.

Noticed-by: Max L <infthi.inbox@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
run_external_diff: use an argv_array for the environment

We currently use static buffers and a static array for
formatting the environment passed to the external diff.
There's nothing wrong in the code, but it is much easier to
verify that it is correct if we use a dynamic argv_array.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
run_external_diff: clean up error handling

When the external diff reports an error, we try to clean up
and die. However, we can make this process a bit simpler:

  1. We do not need to bother freeing memory, since we are
     about to exit.  Nor do we need to clean up our
     tempfiles, since the atexit() handler will do it for
     us. So we can die as soon as we see the error.

  3. We can just call die() rather than fprintf/exit. This
     does technically change our exit code, but the exit
     code of "1" is not meaningful here. In fact, it is
     probably wrong, since "1" from diff usually means
     "completed successfully, but there were differences".

And while we're there, we can mark the error message for
translation, and drop the full stop at the end to make it
more like our other messages.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
run_external_diff: drop fflush(NULL)

This fflush was added in d5535ec (Use run_command() to spawn
external diff programs instead of fork/exec., 2007-10-19),
because flushing buffers before forking is a good habit.

But later, 7d0b18a (Add output flushing before fork(),
2008-08-04) added it to the generic run-command interface,
meaning that our flush here is redundant.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
run_external_diff: hoist common bits out of conditional

Whether we have diff_filespecs to give to the diff command
or not, we always are going to run the program and pass it
the pathname. Let's pull that duplicated part out of the
conditional to make it more obvious.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
run_external_diff: refactor cmdline setup logic

The current logic makes it hard to see what gets put onto
the command line in which cases. Pulling out a helper
function lets us see that we have two sets of file data, and
the second set either uses the original name, or the "other"
renamed/copy name.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'jk/external-diff-use-argv-array' into jch

* 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
  run_external_diff: use an argv_array for the command line