builtin/diff-index: learn --merge-base

There is currently no easy way to take the diff between the working tree
or index and the merge base between an arbitrary commit and HEAD. Even
diff's `...` notation doesn't allow this because it only works between
commits. However, the ability to do this would be desirable to a user
who would like to see all the changes they've made on a branch plus
uncommitted changes without taking into account changes made in the
upstream branch.

Teach diff-index and diff (with one commit) the --merge-base option
which allows a user to use the merge base of a commit and HEAD as the
"before" side.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 14 insertions, 1 deletions

diff --git a/diff-lib.c b/diff-lib.c
index 468e3fe854..c578560997 100644
--- a/diff-lib.c
+++ b/diff-lib.c
@@ -561,13 +561,26 @@ int run_diff_index(struct rev_info *revs, unsigned int option)
 {
 	struct object_array_entry *ent;
 	int cached = !!(option & DIFF_INDEX_CACHED);
+	int merge_base = !!(option & DIFF_INDEX_MERGE_BASE);
+	struct object_id oid;
+	const char *name;
+	char merge_base_hex[GIT_MAX_HEXSZ + 1];
 
 	if (revs->pending.nr != 1)
 		BUG("run_diff_index must be passed exactly one tree");
 
 	trace_performance_enter();
 	ent = revs->pending.objects;
-	if (diff_cache(revs, &ent->item->oid, ent->name, cached))
+
+	if (merge_base) {
+		diff_get_merge_base(revs, &oid);
+		name = oid_to_hex_r(merge_base_hex, &oid);
+	} else {
+		oidcpy(&oid, &ent->item->oid);
+		name = ent->name;
+	}
+
+	if (diff_cache(revs, &oid, name, cached))
 		exit(128);
 
 	diff_set_mnemonic_prefix(&revs->diffopt, "c/", cached ? "i/" : "w/");