resolve-undo: record resolved conflicts in a new index extension section

When resolving a conflict using "git add" to create a stage #0 entry, or
"git rm" to remove entries at higher stages, remove_index_entry_at()
function is eventually called to remove unmerged (i.e. higher stage)
entries from the index.  Introduce a "resolve_undo_info" structure and
keep track of the removed cache entries, and save it in a new index
extension section in the index_state.

Operations like "read-tree -m", "merge", "checkout [-m] <branch>" and
"reset" are signs that recorded information in the index is no longer
necessary.  The data is removed from the index extension when operations
start; they may leave conflicted entries in the index, and later user
actions like "git add" will record their conflicted states afresh.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-read-tree.c

@@ -13,6 +13,7 @@
 #include "dir.h"
 #include "builtin.h"
 #include "parse-options.h"
+#include "resolve-undo.h"
 
 static int nr_trees;
 static struct tree *trees[MAX_UNPACK_TREES];
@@ -122,6 +123,7 @@ int cmd_read_tree(int argc, const char **argv, const char *unused_prefix)
 			die("You need to resolve your current index first");
 		stage = opts.merge = 1;
 	}
+	resolve_undo_clear();
 
 	for (i = 0; i < argc; i++) {
 		const char *arg = argv[i];