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>

1 files changed, 3 insertions, 1 deletions

diff --git a/builtin-merge.c b/builtin-merge.c
index 6cb804b6ce..6bc2f7af08 100644
--- a/builtin-merge.c
+++ b/builtin-merge.c
@@ -24,6 +24,7 @@
 #include "rerere.h"
 #include "help.h"
 #include "merge-recursive.h"
+#include "resolve-undo.h"
 
 #define DEFAULT_TWOHEAD (1<<0)
 #define DEFAULT_OCTOPUS (1<<1)
@@ -604,6 +605,7 @@ static int try_merge_strategy(const char *strategy, struct commit_list *common,
 		discard_cache();
 		if (read_cache() < 0)
 			die("failed to read the cache");
+		resolve_undo_clear();
 		return ret;
 	}
 }
@@ -851,7 +853,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	if (read_cache_unmerged())
 		die("You are in the middle of a conflicted merge."
 				" (index unmerged)");
-
+	resolve_undo_clear();
 	/*
 	 * Check if we are _not_ on a detached HEAD, i.e. if there is a
 	 * current branch.