stash show: teach --include-untracked and --only-untracked

Stash entries can be made with untracked files via
`git stash push --include-untracked`. However, because the untracked
files are stored in the third parent of the stash entry and not the
stash entry itself, running `git stash show` does not include the
untracked files as part of the diff.

With --include-untracked, untracked paths, which are recorded in the
third-parent if it exists, are shown in addition to the paths that have
modifications between the stash base and the working tree in the stash.

It is possible to manually craft a malformed stash entry where duplicate
untracked files in the stash entry will mask tracked files. We detect
and error out in that case via a custom unpack_trees() callback:
stash_worktree_untracked_merge().

Also, teach stash the --only-untracked option which only shows the
untracked files of a stash entry. This is similar to `git show stash^3`
but it is nice to provide a convenient abstraction for it so that users
do not have to think about the underlying implementation.

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

1 files changed, 22 insertions, 0 deletions

diff --git a/unpack-trees.c b/unpack-trees.c
index af6e9b9c2f..dde8c320c5 100644
--- a/unpack-trees.c
+++ b/unpack-trees.c
@@ -2566,3 +2566,25 @@ int oneway_merge(const struct cache_entry * const *src,
 	}
 	return merged_entry(a, old, o);
 }
+
+/*
+ * Merge worktree and untracked entries in a stash entry.
+ *
+ * Ignore all index entries. Collapse remaining trees but make sure that they
+ * don't have any conflicting files.
+ */
+int stash_worktree_untracked_merge(const struct cache_entry * const *src,
+				   struct unpack_trees_options *o)
+{
+	const struct cache_entry *worktree = src[1];
+	const struct cache_entry *untracked = src[2];
+
+	if (o->merge_size != 2)
+		BUG("invalid merge_size: %d", o->merge_size);
+
+	if (worktree && untracked)
+		return error(_("worktree and untracked commit have duplicate entries: %s"),
+			     super_prefixed(worktree->name));
+
+	return merged_entry(worktree ? worktree : untracked, NULL, o);
+}