builtin-am: handle stray state directory

Should git-am terminate unexpectedly between the point where the state
directory is created, but the "next" and "last" files are not written
yet, a stray state directory will be left behind.

As such, since b141f3c (am: handle stray $dotest directory, 2013-06-15),
git-am.sh explicitly recognizes such a stray directory, and allows the
user to remove it with am --abort.

Re-implement this feature in builtin/am.c.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pyokagan

builtin/am.c

@@ -1530,6 +1530,23 @@ int cmd_am(int argc, const char **argv, const char *prefix)
 		struct argv_array paths = ARGV_ARRAY_INIT;
 		int i;
 
+		/*
+		 * Handle stray state directory in the independent-run case. In
+		 * the --rebasing case, it is up to the caller to take care of
+		 * stray directories.
+		 */
+		if (file_exists(state.dir) && !state.rebasing) {
+			if (resume == RESUME_ABORT) {
+				am_destroy(&state);
+				am_state_release(&state);
+				return 0;
+			}
+
+			die(_("Stray %s directory found.\n"
+				"Use \"git am --abort\" to remove it."),
+				state.dir);
+		}
+
 		if (resume)
 			die(_("Resolve operation not in progress, we are not resuming."));