merge-recursive: fix assumption that head tree being merged is HEAD

`git merge-recursive` does a three-way merge between user-specified trees
base, head, and remote.  Since the user is allowed to specify head, we can
not necesarily assume that head == HEAD.

Modify index_has_changes() to take an extra argument specifying the tree
to compare against.  If NULL, it will compare to HEAD.  We then use this
from merge-recursive to make sure we compare to the user-specified head.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 7 insertions, 4 deletions

diff --git a/cache.h b/cache.h
index 169c809..fad10c3 100644
--- a/cache.h
+++ b/cache.h
@@ -218,6 +218,7 @@ struct cache_entry {
 /* Forward structure decls */
 struct pathspec;
 struct child_process;
+struct tree;
 
 /*
  * Copy the sha1 and stat state of a cache entry from one to
@@ -627,12 +628,14 @@ extern void move_index_extensions(struct index_state *dst, struct index_state *s
 extern int unmerged_index(const struct index_state *);
 
 /**
- * Returns 1 if istate differs from HEAD, 0 otherwise.  When on an unborn
- * branch, returns 1 if there are entries in istate, 0 otherwise.  If an
- * strbuf is provided, the space-separated list of files that differ will
- * be appended to it.
+ * Returns 1 if istate differs from tree, 0 otherwise.  If tree is NULL,
+ * compares istate to HEAD.  If tree is NULL and on an unborn branch,
+ * returns 1 if there are entries in istate, 0 otherwise.  If an strbuf is
+ * provided, the space-separated list of files that differ will be appended
+ * to it.
  */
 extern int index_has_changes(const struct index_state *istate,
+			     struct tree *tree,
 			     struct strbuf *sb);
 
 extern int verify_path(const char *path, unsigned mode);