Make git rebase work with --rebase-merges and --exec

It was reported via IRC that the exec lines are inserted in the wrong spots when using --rebase-merges.

The reason is that we used a simple, incorrect implementation that happened to work as long as the generated todo list only contains pick, fixup and squash commands. Which is not the case with--rebase-merges.

Fix this issue by using a correct implementation instead, that even takes into account merge commands in the --rebase-merges mode.

Changes since v1:

* Replaced the "look-ahead" design by a "keep looking" one: instead of having a nested loop that looks for the end of the fixup/squash chain, we continue the loop, delaying the insertion until we know where the fixup/squash chain ends, if any.

Johannes Schindelin (2):
t3430: demonstrate what -r, --autosquash & --exec should do rebase --exec: make it work with --rebase-merges

 sequencer.c              | 42 +++++++++++++++++++++++++++++-----------
 t/t3430-rebase-merges.sh | 17 ++++++++++++++++
 2 files changed, 48 insertions(+), 11 deletions(-)

base-commit: 1d89318c48d233d52f1db230cf622935ac3c69fa
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-13%2Fdscho%2Frebase-merges-and-exec-commands-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-13/dscho/rebase-merges-and-exec-commands-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/13

Range-diff vs v2:

 1:  1d82eb450 = 1:  1d82eb450 t3430: demonstrate what -r, --autosquash & --exec should do
 2:  7ca441a89 ! 2:  b436f67ba rebase --exec: make it work with --rebase-merges
     @@ -22,6 +22,11 @@
          `pick` lines, skip any fixup/squash chains, and then insert the `exec`
          line. Lather, rinse, repeat.
      
     +    Note: we take pains to insert *before* comment lines whenever possible,
     +    as empty commits are represented by commented-out pick lines (and we
     +    want to insert a preceding pick's exec line *before* such a line, not
     +    afterward).
     +
          While at it, also add `exec` lines after `merge` commands, because they
          are similar in spirit to `pick` commands: they add new commits.
      
     @@ -81,9 +86,13 @@
      +			insert = i + 1;
       	}
       
     - 	/* append final <commands> */
     +-	/* append final <commands> */
      -	strbuf_add(buf, commands, commands_len);
     -+	if (insert >= 0 || !offset)
     ++	/* insert or append final <commands> */
     ++	if (insert >= 0 && insert < todo_list.nr)
     ++		strbuf_insert(buf, todo_list.items[insert].offset_in_buf +
     ++			      offset, commands, commands_len);
     ++	else if (insert >= 0 || !offset)
      +		strbuf_add(buf, commands, commands_len);
       
       	i = write_message(buf->buf, buf->len, todo_file, 0);

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