diff: add --slide-down option to shift ambiguous hunks downward

When an inserted block is bounded by lines identical to the
surrounding context (e.g., "}") the diff slider can place
the hunk at any position within the slidable range.

Add --slide-down (and diff.slideDown config) to leave ambiguous hunks
at the furthest-down position, so that insertions appear appended
after the existing context rather than prepended before it.  This
results in a more readable git diff, without adding any extra lines
to the diff.

Signed-off-by:  Charles G Waldman <cgw@alum.mit.edu>

Author: charlesgwaldman

diff.c

@@ -55,6 +55,7 @@
 
 static int diff_detect_rename_default;
 static int diff_indent_heuristic = 1;
+static int diff_slide_down;
 static int diff_rename_limit_default = 1000;
 static int diff_suppress_blank_empty;
 static enum git_colorbool diff_use_color_default = GIT_COLOR_UNKNOWN;
@@ -289,6 +290,8 @@ int git_diff_heuristic_config(const char *var, const char *value,
 {
 	if (!strcmp(var, "diff.indentheuristic"))
 		diff_indent_heuristic = git_config_bool(var, value);
+	if (!strcmp(var, "diff.slidedown"))
+		diff_slide_down = git_config_bool(var, value);
 	return 0;
 }
 
@@ -4861,6 +4864,8 @@ void repo_diff_setup(struct repository *r, struct diff_options *options)
 	options->xdl_opts |= diff_algorithm;
 	if (diff_indent_heuristic)
 		DIFF_XDL_SET(options, INDENT_HEURISTIC);
+	if (diff_slide_down)
+		DIFF_XDL_SET(options, SLIDE_DOWN);
 
 	options->orderfile = xstrdup_or_null(diff_order_file_cfg);
 
@@ -5910,6 +5915,9 @@ struct option *add_diff_options(const struct option *opts,
 		OPT_BIT(0, "indent-heuristic", &options->xdl_opts,
 			N_("heuristic to shift diff hunk boundaries for easy reading"),
 			XDF_INDENT_HEURISTIC),
+		OPT_BIT(0, "slide-down", &options->xdl_opts,
+			N_("slide ambiguous diff hunks downward (append after context)"),
+			XDF_SLIDE_DOWN),
 		OPT_CALLBACK_F(0, "patience", options, NULL,
 			       N_("generate diff using the \"patience diff\" algorithm"),
 			       PARSE_OPT_NONEG | PARSE_OPT_NOARG,

xdiff/xdiff.h

@@ -47,6 +47,7 @@ extern "C" {
 #define XDF_DIFF_ALG(x) ((x) & XDF_DIFF_ALGORITHM_MASK)
 
 #define XDF_INDENT_HEURISTIC (1 << 23)
+#define XDF_SLIDE_DOWN (1 << 24)
 
 /* xdemitconf_t.flags */
 #define XDL_EMIT_FUNCNAMES (1 << 0)

xdiff/xdiffi.c

@@ -857,6 +857,15 @@ int xdl_change_compact(xdfile_t *xdf, xdfile_t *xdfo, long flags) {
 
 		if (g.end == earliest_end) {
 			/* no shifting was possible */
+		} else if (flags & XDF_SLIDE_DOWN) {
+			/*
+			 * --slide-down: the group is already at the
+			 * furthest-down position. Leave it there.
+			 * This produces diffs where ambiguous hunks
+			 * (e.g., duplicate delimiter lines) appear
+			 * as appended after existing context rather
+			 * than inserted before it.
+			 */
 		} else if (end_matching_other != -1) {
 			/*
 			 * Move the possibly merged group of changes back to