Fix region regexes incorrectly matching ^$ at end of line

Because no flags were set for regex matching, the regex engine was
assuming that the subject string past-the-end matched a end-of-line.
As the subject string already ended with a \n character, the regex
engine processing of the "past-the-end" position would match '^$'
as ^ matched past the existing \n and $ matched the assumed
end-of-line.

Fixes #3799

src/highlighters.cc

@@ -1744,7 +1744,8 @@ void insert_matches(const Buffer& buffer, RegexMatchList& matches, const Regex&
     for (auto line = range.begin; line < range.end; ++line)
     {
         const StringView l = buffer[line];
-        for (auto&& m : RegexIterator{l.begin(), l.end(), vm})
+        const auto flags = RegexExecFlags::NotEndOfLine; // buffer line already ends with \n
+        for (auto&& m : RegexIterator{l.begin(), l.end(), vm, flags})
         {
             const bool with_capture = capture and m[1].matched and
                                       m[0].second - m[0].first < std::numeric_limits<uint16_t>::max();

test/regression/3799-incorrect-region-match/cmd

@@ -0,0 +1 @@
+

test/regression/3799-incorrect-region-match/in

@@ -0,0 +1,4 @@
+print << "";
+part of heredoc
+
+not part of heredoc

test/regression/3799-incorrect-region-match/rc

@@ -0,0 +1,2 @@
+add-highlighter global/regions regions
+add-highlighter global/regions/heredoc region -match-capture <<\h*"(\w*)" ^(\w*)$ fill red

test/regression/3799-incorrect-region-match/script

@@ -0,0 +1,2 @@
+ui_out '{ "jsonrpc": "2.0", "method": "set_ui_options", "params": [{}] }'
+ui_out '{ "jsonrpc": "2.0", "method": "draw", "params": [[[{ "face": { "fg": "black", "bg": "white", "attributes": [] }, "contents": "p" }, { "face": { "fg": "default", "bg": "default", "attributes": [] }, "contents": "rint " }, { "face": { "fg": "red", "bg": "default", "attributes": [] }, "contents": "<< \"\";\u000a" }], [{ "face": { "fg": "red", "bg": "default", "attributes": [] }, "contents": "part of heredoc\u000a" }], [{ "face": { "fg": "default", "bg": "default", "attributes": [] }, "contents": "\u000a" }], [{ "face": { "fg": "default", "bg": "default", "attributes": [] }, "contents": "not part of heredoc\u000a" }]], { "fg": "default", "bg": "default", "attributes": [] }, { "fg": "blue", "bg": "default", "attributes": [] }] }'