resolve-undo: allow plumbing to clear the information

At the Porcelain level, operations such as merge that populate an
initially cleanly merged index with conflicted entries clear the
resolve-undo information upfront.  Give scripted Porcelains a way
to do the same, by implementing "update-index --clear-resolve-info".

With this, a scripted Porcelain may "update-index --clear-resolve-info"
first and repeatedly run "update-index --cacheinfo" to stuff unmerged
entries to the index, to be resolved by the user with "git add" and
stuff.

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

builtin-update-index.c

@@ -9,6 +9,7 @@
 #include "tree-walk.h"
 #include "builtin.h"
 #include "refs.h"
+#include "resolve-undo.h"
 
 /*
  * Default to not allowing changes to the list of files. The
@@ -703,6 +704,10 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 				verbose = 1;
 				continue;
 			}
+			if (!strcmp(path, "--clear-resolve-undo")) {
+				resolve_undo_clear();
+				continue;
+			}
 			if (!strcmp(path, "-h") || !strcmp(path, "--help"))
 				usage(update_index_usage);
 			die("unknown option %s", path);

t/t2030-unresolve-info.sh

@@ -85,4 +85,16 @@ test_expect_success 'rm records reset clears' '
 	check_resolve_undo discarded
 '
 
+test_expect_success 'plumbing clears' '
+	prime_resolve_undo &&
+	test_tick &&
+	git commit -m merged &&
+	echo committing keeps &&
+	check_resolve_undo kept file initial:file second:file third:file &&
+
+	echo plumbing clear &&
+	git update-index --clear-resolve-undo &&
+	check_resolve_undo cleared
+'
+
 test_done