Accept docstrings with last line not fully indented

When trimming indent, the last line, if only containing
whitespaces does not need to match the indent, so that
this indentation style works:

    -docstring %{
        indented string
    }

rc/tools/ctags.kak

@@ -26,10 +26,11 @@ define-command -params ..1 \
             fi
             cat "$namecache"
         done} \
-    -docstring %{ctags-search [<symbol>]: jump to a symbol's definition
-If no symbol is passed then the current selection is used as symbol name} \
-    ctags-search \
-    %[ evaluate-commands %sh[
+    -docstring %{
+        ctags-search [<symbol>]: jump to a symbol's definition
+        If no symbol is passed then the current selection is used as symbol name
+    } \
+    ctags-search %[ evaluate-commands %sh[
         realpath() { ( cd "$(dirname "$1")"; printf "%s/%s\n" "$(pwd -P)" "$(basename "$1")" ) }
         export tagname="${1:-${kak_selection}}"
         eval "set -- $kak_quoted_opt_ctagsfiles"

src/string_utils.cc

@@ -23,31 +23,25 @@ String trim_indent(StringView str)
 {
     if (str.empty())
         return {};
-    else if (str[0_byte] != '\n')
-        return trim_whitespaces(str).str();
 
-    str = str.substr(1_byte);
-    const CharCount docstring_length = str.char_length();
+    if (str[0_byte] == '\n')
+        str = str.substr(1_byte);
+    while (not str.empty() and is_blank(str.back()))
+        str = str.substr(0, str.length() - 1);
 
-    CharCount level_indent = 0;
-    while (level_indent < docstring_length
-           and is_horizontal_blank(str[level_indent]))
-        level_indent++;
+    utf8::iterator it{str.begin(), str};
+    while (it != str.end() and is_horizontal_blank(*it))
+        it++;
 
-    if (level_indent >= docstring_length or not level_indent)
-        return trim_whitespaces(str).str();
+    const StringView indent{str.begin(), it.base()};
+    return accumulate(str | split_after<StringView>('\n') | transform([&](auto&& line) {
+            if (line == "\n")
+                return line;
+            else if (not prefix_match(line, indent))
+                throw runtime_error("inconsistent indentation in the string");
 
-    const auto str_indent = str.substr(0, level_indent);
-    auto s = str | split<StringView>('\n') | transform([&](auto&& line) {
-        if (line.empty())
-            return line;
-        else if (not prefix_match(line, str_indent))
-            throw runtime_error("inconsistent indentation in the string");
-
-        return line.substr(str_indent.char_length());
-    });
-
-    return trim_whitespaces(join(s, '\n', false)).str();
+            return line.substr(indent.length());
+        }), String{}, [](String& s, StringView l) -> decltype(auto) { return s += l; });
 }
 
 String escape(StringView str, StringView characters, char escape)
@@ -414,6 +408,7 @@ UnitTest test_string{[]()
     kak_assert(trim_indent("\nno-indent") == "no-indent");
     kak_assert(trim_indent("\n  indent\n  indent") == "indent\nindent");
     kak_assert(trim_indent("\n  indent\n    indent") == "indent\n  indent");
+    kak_assert(trim_indent("\n  indent\n  indent\n   ") == "indent\nindent");
 
     kak_expect_throw(runtime_error, trim_indent("\n  indent\nno-indent"));