[PATCH 1/2] rebase: help user when dying with preserve-merges`

["Philip Oakley via GitGitGadget" <gitgitgadget@gmail.com>]

From: Philip Oakley <philipoakley@iee.email>

Git will die if a "rebase --preserve-merges" is in progress.
Users cannot --quit, --abort or --continue the rebase.

This sceario can occur if the user updates their Git, or switches
to another newer version, after starting a preserve-merges rebase,
commonly via the pull setting.

One trigger is an unexpectedly difficult to resolve conflict, as
reported on the `git-users` group.
(https://groups.google.com/g/git-for-windows/c/3jMWbBlXXHM)

Tell the user the cause, i.e. the existence of the directory.
The problem must be resolved manually, `git rebase --<option>`
commands will die, or the user must downgrade. Also, note that
the deleted options are no longer shown in the documentation.

Signed-off-by: Philip Oakley <philipoakley@iee.email>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
---
 builtin/rebase.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index d858add3fe8..07221d0ae41 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1181,7 +1181,9 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "%s/rewritten", merge_dir());
 		if (is_directory(buf.buf)) {
-			die("`rebase -p` is no longer supported");
+			die("`rebase --preserve-merges` (-p) is no longer supported.\n"
+			"You still have a `.git/rebase-merge/rewritten` directory, \n"
+			"indicating a `rebase preserve-merge` is still in progress.\n");
 		} else {
 			strbuf_reset(&buf);
 			strbuf_addf(&buf, "%s/interactive", merge_dir());
-- 
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