revert: Introduce --reset to remove sequencer state

To explicitly remove the sequencer state for a fresh cherry-pick or
revert invocation, introduce a new subcommand called "--reset" to
remove the sequencer state.

Take the opportunity to publicly expose the sequencer paths, and a
generic function called "remove_sequencer_state" that various git
programs can use to remove the sequencer state in a uniform manner;
"git reset" uses it later in this series.  Introducing this public API
is also in line with our long-term goal of eventually factoring out
functions from revert.c into a generic commit sequencer.

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

sequencer.h

@@ -0,0 +1,20 @@
+#ifndef SEQUENCER_H
+#define SEQUENCER_H
+
+#define SEQ_DIR		"sequencer"
+#define SEQ_OLD_DIR	"sequencer-old"
+#define SEQ_HEAD_FILE	"sequencer/head"
+#define SEQ_TODO_FILE	"sequencer/todo"
+#define SEQ_OPTS_FILE	"sequencer/opts"
+
+/*
+ * Removes SEQ_OLD_DIR and renames SEQ_DIR to SEQ_OLD_DIR, ignoring
+ * any errors.  Intended to be used by 'git reset'.
+ *
+ * With the aggressive flag, it additionally removes SEQ_OLD_DIR,
+ * ignoring any errors.  Inteded to be used by the sequencer's
+ * '--reset' subcommand.
+ */
+void remove_sequencer_state(int aggressive);
+
+#endif