am: handle stray $dotest directory

The following bug has been observed:

  $ git am  # no input file
  ^C
  $ git am --abort
  Resolve operation not in progress, we are not resuming.

This happens because the following test fails:

  test -d "$dotest" && test -f "$dotest/last" && test -f "$dotest/next"

and the codepath for an "am in-progress" is not executed.  It falls back
to the codepath that treats this as a "fresh execution".  Before
rr/rebase-autostash, this condition was

  test -d "$dotest"

It would incorrectly execute the "normal" am --abort codepath:

  git read-tree --reset -u HEAD ORIG_HEAD
  git reset ORIG_HEAD

by incorrectly assuming that an am is "in progress" (i.e. ORIG_HEAD
etc. was written during the previous execution).

Notice that

  $ git am
  ^C

executes nothing of significance, is equivalent to

  $ mkdir .git/rebase-apply

Therefore, the correct solution is to treat .git/rebase-apply as a
"stray directory" and remove it on --abort in the fresh-execution
codepath.  Also ensure that we're not called with --rebasing from
git-rebase--am.sh; in that case, it is the responsibility of the caller
to handle and stray directories.

While at it, tell the user to run "git am --abort" to get rid of the
stray $dotest directory, if she attempts anything else.

Reported-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-am.sh

@@ -506,6 +506,23 @@ then
 	esac
 	rm -f "$dotest/dirtyindex"
 else
+	# Possible stray $dotest directory in the independent-run
+	# case; in the --rebasing case, it is upto the caller
+	# (git-rebase--am) to take care of stray directories.
+	if test -d "$dotest" && test -z "$rebasing"
+	then
+		case "$skip,$resolved,$abort" in
+		,,t)
+			rm -fr "$dotest"
+			exit 0
+			;;
+		*)
+			die "$(eval_gettext "Stray \$dotest directory found.
+Use \"git am --abort\" to remove it.")"
+			;;
+		esac
+	fi
+
 	# Make sure we are not given --skip, --resolved, nor --abort
 	test "$skip$resolved$abort" = "" ||
 		die "$(gettext "Resolve operation not in progress, we are not resuming.")"

t/t4150-am.sh

@@ -363,6 +363,12 @@ test_expect_success 'am --skip works' '
 	test_cmp expected another
 '
 
+test_expect_success 'am --abort removes a stray directory' '
+	mkdir .git/rebase-apply &&
+	git am --abort &&
+	test_path_is_missing .git/rebase-apply
+'
+
 test_expect_success 'am --resolved works' '
 	echo goodbye >expected &&
 	rm -fr .git/rebase-apply &&