indent selector: When line is empty, find indent from surrounding lines

Look for the first non empty line preceeding the current line, or
if not found, the first non empty line following it.

Fixes #1904

src/selectors.cc

@@ -654,6 +654,17 @@ select_indent(const Context& context, const Selection& selection,
         return indent;
     };
 
+    auto get_current_indent = [&](const Buffer& buffer, LineCount line, int tabstop)
+    {
+        for (auto l = line; l >= 0; --l)
+            if (buffer[l] != "\n"_sv)
+                return get_indent(buffer[l], tabstop);
+        for (auto l = line+1; l < buffer.line_count(); ++l)
+            if (buffer[l] != "\n"_sv)
+                return get_indent(buffer[l], tabstop);
+        return 0_char;
+    };
+
     auto is_only_whitespaces = [](StringView str) {
         auto it = str.begin();
         skip_while(it, str.end(),
@@ -667,8 +678,8 @@ select_indent(const Context& context, const Selection& selection,
     auto& buffer = context.buffer();
     int tabstop = context.options()["tabstop"].get<int>();
     auto pos = selection.cursor();
-    LineCount line = pos.line;
-    auto indent = get_indent(buffer[line], tabstop);
+    const LineCount line = pos.line;
+    const auto indent = get_current_indent(buffer, line, tabstop);
 
     LineCount begin_line = line - 1;
     if (to_begin)

test/regression/1904-select-empty-line-indent-gets-whole-buffer/cmd

@@ -0,0 +1 @@
+2j<a-i>i

test/regression/1904-select-empty-line-indent-gets-whole-buffer/in

@@ -0,0 +1,4 @@
+foo():
+    foo
+
+    bar

test/regression/1904-select-empty-line-indent-gets-whole-buffer/selections

@@ -0,0 +1,4 @@
+    foo
+
+    bar
+