stash: save the workdir file when deleted in index

When stashing the workdir tree, examine the index as well.  Using
a mechanism similar to `git_diff_tree_to_workdir_with_index`
allows us to determine that a file was added in the index and
subsequently modified in the working directory.  Without examining
the index, we would erroneously believe that this file was
untracked and fail to include it in the working directory tree.

Use a slightly modified `git_diff_tree_to_workdir_with_index` in
order to avoid some of the behavior custom to `git diff`.  In
particular, be sure to include the working directory side of a
file when it was deleted in the index.

src/diff.h

@@ -133,6 +133,15 @@ typedef git_diff_delta *(*git_diff__merge_cb)(
 extern int git_diff__merge(
 	git_diff *onto, const git_diff *from, git_diff__merge_cb cb);
 
+extern git_diff_delta *git_diff__merge_like_cgit(
+	const git_diff_delta *a,
+	const git_diff_delta *b,
+	git_pool *pool);
+
+/* Duplicate a `git_diff_delta` out of the `git_pool` */
+extern git_diff_delta *git_diff__delta_dup(
+	const git_diff_delta *d, git_pool *pool);
+
 /*
  * Sometimes a git_diff_file will have a zero size; this attempts to
  * fill in the size without loading the blob if possible.  If that is

src/diff_tform.c

@@ -15,7 +15,7 @@
 #include "fileops.h"
 #include "config.h"
 
-static git_diff_delta *diff_delta__dup(
+git_diff_delta *git_diff__delta_dup(
 	const git_diff_delta *d, git_pool *pool)
 {
 	git_diff_delta *delta = git__malloc(sizeof(git_diff_delta));
@@ -46,7 +46,7 @@ fail:
 	return NULL;
 }
 
-static git_diff_delta *diff_delta__merge_like_cgit(
+git_diff_delta *git_diff__merge_like_cgit(
 	const git_diff_delta *a,
 	const git_diff_delta *b,
 	git_pool *pool)
@@ -67,16 +67,16 @@ static git_diff_delta *diff_delta__merge_like_cgit(
 
 	/* If one of the diffs is a conflict, just dup it */
 	if (b->status == GIT_DELTA_CONFLICTED)
-		return diff_delta__dup(b, pool);
+		return git_diff__delta_dup(b, pool);
 	if (a->status == GIT_DELTA_CONFLICTED)
-		return diff_delta__dup(a, pool);
+		return git_diff__delta_dup(a, pool);
 
 	/* if f2 == f3 or f2 is deleted, then just dup the 'a' diff */
 	if (b->status == GIT_DELTA_UNMODIFIED || a->status == GIT_DELTA_DELETED)
-		return diff_delta__dup(a, pool);
+		return git_diff__delta_dup(a, pool);
 
 	/* otherwise, base this diff on the 'b' diff */
-	if ((dup = diff_delta__dup(b, pool)) == NULL)
+	if ((dup = git_diff__delta_dup(b, pool)) == NULL)
 		return NULL;
 
 	/* If 'a' status is uninteresting, then we're done */
@@ -109,7 +109,7 @@ static git_diff_delta *diff_delta__merge_like_cgit(
 	return dup;
 }
 
-int git_diff__merge_deltas(
+int git_diff__merge(
 	git_diff *onto, const git_diff *from, git_diff__merge_cb cb)
 {
 	int error = 0;
@@ -146,10 +146,10 @@ int git_diff__merge_deltas(
 			STRCMP_CASESELECT(ignore_case, o->old_file.path, f->old_file.path);
 
 		if (cmp < 0) {
-			delta = diff_delta__dup(o, &onto_pool);
+			delta = git_diff__delta_dup(o, &onto_pool);
 			i++;
 		} else if (cmp > 0) {
-			delta = diff_delta__dup(f, &onto_pool);
+			delta = git_diff__delta_dup(f, &onto_pool);
 			j++;
 		} else {
 			const git_diff_delta *left = reversed ? f : o;
@@ -196,7 +196,7 @@ int git_diff__merge_deltas(
 
 int git_diff_merge(git_diff *onto, const git_diff *from)
 {
-	return git_diff__merge_deltas(onto, from, diff_delta__merge_like_cgit);
+	return git_diff__merge(onto, from, git_diff__merge_like_cgit);
 }
 
 int git_diff_find_similar__hashsig_for_file(
@@ -347,7 +347,7 @@ static int insert_delete_side_of_split(
 	git_diff *diff, git_vector *onto, const git_diff_delta *delta)
 {
 	/* make new record for DELETED side of split */
-	git_diff_delta *deleted = diff_delta__dup(delta, &diff->pool);
+	git_diff_delta *deleted = git_diff__delta_dup(delta, &diff->pool);
 	GITERR_CHECK_ALLOC(deleted);
 
 	deleted->status = GIT_DELTA_DELETED;

src/stash.c

@@ -22,6 +22,7 @@
 #include "signature.h"
 #include "iterator.h"
 #include "merge.h"
+#include "diff.h"
 
 static int create_error(int error, const char *msg)
 {
@@ -292,6 +293,25 @@ cleanup:
 	return error;
 }
 
+static git_diff_delta *stash_delta_merge(
+	const git_diff_delta *a,
+	const git_diff_delta *b,
+	git_pool *pool)
+{
+	/* Special case for stash: if a file is deleted in the index, but exists
+	 * in the working tree, we need to stash the workdir copy for the workdir.
+	 */
+	if (a->status == GIT_DELTA_DELETED && b->status == GIT_DELTA_UNTRACKED) {
+		git_diff_delta *dup = git_diff__delta_dup(b, pool);
+
+		if (dup)
+			dup->status = GIT_DELTA_MODIFIED;
+		return dup;
+	}
+
+	return git_diff__merge_like_cgit(a, b, pool);
+}
+
 static int build_workdir_tree(
 	git_tree **tree_out,
 	git_index *index,
@@ -299,17 +319,19 @@ static int build_workdir_tree(
 {
 	git_repository *repo = git_index_owner(index);
 	git_tree *b_tree = NULL;
-	git_diff *diff = NULL;
+	git_diff *diff = NULL, *idx_to_wd = NULL;
 	git_diff_options opts = GIT_DIFF_OPTIONS_INIT;
 	struct stash_update_rules data = {0};
 	int error;
 
-	opts.flags = GIT_DIFF_IGNORE_SUBMODULES;
+	opts.flags = GIT_DIFF_IGNORE_SUBMODULES | GIT_DIFF_INCLUDE_UNTRACKED;
 
 	if ((error = git_commit_tree(&b_tree, b_commit)) < 0)
 		goto cleanup;
 
-	if ((error = git_diff_tree_to_workdir(&diff, repo, b_tree, &opts)) < 0)
+	if ((error = git_diff_tree_to_index(&diff, repo, b_tree, index, &opts)) < 0 ||
+		(error = git_diff_index_to_workdir(&idx_to_wd, repo, index, &opts)) < 0 ||
+		(error = git_diff__merge(diff, idx_to_wd, stash_delta_merge)) < 0)
 		goto cleanup;
 
 	data.include_changed = true;
@@ -320,6 +342,7 @@ static int build_workdir_tree(
 	error = build_tree_from_index(tree_out, index);
 
 cleanup:
+	git_diff_free(idx_to_wd);
 	git_diff_free(diff);
 	git_tree_free(b_tree);