diff: ignore sparse paths in diffstat

The diff_populate_filespec() method is used to describe the diff after a
merge operation is complete. In order to avoid expanding a sparse index,
the reuse_worktree_file() needs to be adapted to ignore files that are
outside of the sparse-checkout cone. The file names and OIDs used for
this check come from the merged tree in the case of the ORT strategy,
not the index, hence the ability to look into these paths without having
already expanded the index.

The work done by reuse_worktree_file() is only an optimization, and
requires the file being on disk for it to be of any value. Thus, it is
safe to exit the method early if we do not expect the file on disk.

Signed-off-by: Derrick Stolee <[email protected]>
Reviewed-by: Elijah Newren <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: derrickstolee

diff.c

@@ -26,6 +26,7 @@
 #include "parse-options.h"
 #include "help.h"
 #include "promisor-remote.h"
+#include "dir.h"
 
 #ifdef NO_FAST_WORKING_DIRECTORY
 #define FAST_WORKING_DIRECTORY 0
@@ -3907,6 +3908,13 @@ static int reuse_worktree_file(struct index_state *istate,
 	if (!want_file && would_convert_to_git(istate, name))
 		return 0;
 
+	/*
+	 * If this path does not match our sparse-checkout definition,
+	 * then the file will not be in the working directory.
+	 */
+	if (!path_in_sparse_checkout(name, istate))
+		return 0;
+
 	len = strlen(name);
 	pos = index_name_pos(istate, name, len);
 	if (pos < 0)