Fix blame when opened from an initial diff view (#1135)

When running "tig show 529182c -- README.adoc", blames were started
on HEAD instead of 529182c.  Fix this by reading the commit line of
the displayed diff.  This also works when multiple commits are shown
("tig show --all").

Using %(commit) instead of parsing the commit line did unfortunately
not work because, because %(commit) can contain the wrong SHA for when
returning from a blame view to a stash view, see test/blame/stash-test.

Sadly, stashes don't contain commit lines, so we need to find out
their SHAs in another way. Once we fix %(commit), we can use it,
but for now this commit adds a workaround to use the old
"view->vid", which seems to be set in the cases where display
the output from "git stash show".

Also, when blame is requested on a context line, use the current
commit rather than its parent.  This does not change behavior since
a context line can never be blamed on the current commit, but this
is consistent with the stage view, which only uses HEAD for deletions
and the index for others.

[ja: add test and commit message]

Co-authored-by: Thomas Koutcher <thomas.koutcher@online.fr>

src/diff.c

@@ -635,6 +635,8 @@ diff_get_lineno(struct view *view, struct line *line, bool old)
 static enum request
 diff_trace_origin(struct view *view, struct line *line)
 {
+	struct line *commit_line = find_prev_line_by_type(view, line, LINE_COMMIT);
+	char id[SIZEOF_REV];
 	struct line *diff = find_prev_line_by_type(view, line, LINE_DIFF_HEADER);
 	struct line *chunk = find_prev_line_by_type(view, line, LINE_DIFF_CHUNK);
 	const char *chunk_data;
@@ -645,7 +647,7 @@ diff_trace_origin(struct view *view, struct line *line)
 	struct blame_header header;
 	struct blame_commit commit;
 
-	if (!diff || !chunk || chunk == line) {
+	if (!diff || !chunk || chunk == line || diff < commit_line) {
 		report("The line to trace must be inside a diff chunk");
 		return REQ_NONE;
 	}
@@ -686,27 +688,25 @@ diff_trace_origin(struct view *view, struct line *line)
 		}
 	}
 
-	if (chunk_marker == '+')
-		string_copy(ref, view->vid);
+	if (commit_line)
+		string_copy_rev_from_commit_line(id, box_text(commit_line));
 	else
-		string_format(ref, "%s^", view->vid);
+		string_copy(id, view->vid);
 
-	if (string_rev_is_null(ref)) {
-		string_ncopy(view->env->file, file, strlen(file));
-		string_copy(view->env->ref, "");
-		view->env->goto_lineno = lineno - 1;
-
-	} else {
-		if (!diff_blame_line(ref, file, lineno, &header, &commit)) {
-			report("Failed to read blame data");
-			return REQ_NONE;
-		}
+	if (chunk_marker == '-')
+		string_format(ref, "%s^", id);
+	else
+		string_copy(ref, id);
 
-		string_ncopy(view->env->file, commit.filename, strlen(commit.filename));
-		string_copy(view->env->ref, header.id);
-		view->env->goto_lineno = header.orig_lineno - 1;
+	if (!diff_blame_line(ref, file, lineno, &header, &commit)) {
+		report("Failed to read blame data");
+		return REQ_NONE;
 	}
 
+	string_ncopy(view->env->file, commit.filename, strlen(commit.filename));
+	string_copy(view->env->ref, header.id);
+	view->env->goto_lineno = header.orig_lineno - 1;
+
 	return REQ_VIEW_BLAME;
 }
 
@@ -791,10 +791,7 @@ diff_request(struct view *view, enum request request, struct line *line)
 		return diff_common_enter(view, request, line);
 
 	case REQ_REFRESH:
-		if (string_rev_is_null(view->vid))
-			refresh_view(view);
-		else
-			reload_view(view);
+		reload_view(view);
 		return REQ_NONE;
 
 	default:

test/blame/initial-diff-test

@@ -0,0 +1,28 @@
+#!/bin/sh
+
+. libtest.sh
+. libgit.sh
+
+steps '
+	:save-display initial-diff.screen
+	:20 # Move to a deleted line.
+	:view-blame
+	:scroll-right
+	:save-display blame-deleted-line.screen
+'
+
+in_work_dir create_repo_from_tgz "$base_dir/files/scala-js-benchmarks.tgz"
+
+test_tig show a1dcf1a
+
+sed -i.old -n /asInstanceOf/p *.screen
+
+assert_equals 'initial-diff.screen' <<EOF
+-  val global = js.Dynamic.global.asInstanceOf[js.Dictionary]
++  val global = js.Dynamic.global.asInstanceOf[js.Dictionary[js.Any]]
+EOF
+
+# Make sure that we find the commit that introduce the deleted line.
+assert_equals 'blame-deleted-line.screen' <<EOF
+0500  17x  val global = js.Dynamic.global.asInstanceOf[js.Dictionary]
+EOF

test/blame/stash-test

@@ -0,0 +1,49 @@
+#!/bin/sh
+
+. libtest.sh
+. libgit.sh
+
+tigrc <<EOF
+set blame-view = text
+EOF
+
+steps '
+	:view-stash
+	:view-diff
+	:move-last-line
+	:move-up
+	:save-display diff.screen
+	:view-blame
+	:save-display blame1.screen
+	:view-close
+	:view-blame
+	:save-display blame2.screen
+'
+
+git_init
+
+test_setup_work_dir()
+{
+	echo "original line" > file
+	git add file
+	git commit -m "Initial commit"
+	echo "changed line" > file
+	git stash
+}
+
+LINES=3 test_tig
+
+assert_equals 'diff.screen' <<EOF
+-original line
+[diff] Changes to 'file' - line 9 of 10  90%
+EOF
+
+assert_equals 'blame1.screen' <<EOF
+original line
+[blame] file - line 1 of 1 100%
+EOF
+
+assert_equals 'blame2.screen' <<EOF
+original line
+[blame] file - line 1 of 1 100%
+EOF