[gdb/cli] Don't let python colorize strip leading newlines

Consider the test-case gdb.base/async.exp.  Using the executable, I run to
main, and land on a line advertised as line 26:
...
$ gdb outputs/gdb.base/async/async -ex start
Reading symbols from outputs/gdb.base/async/async...
Temporary breakpoint 1 at 0x4004e4: file gdb.base/async.c, line 26.
Starting program: outputs/gdb.base/async/async

Temporary breakpoint 1, main () at gdb.base/async.c:26
26       y = foo ();
...

But actually, the line turns out to be line 28:
...
$ cat -n gdb.base/async.c
    ...
    26   y = 2;
    27   z = 9;
    28   y = foo ();
...

This is caused by the following: the python colorizer initializes the lexer
with default options (no second argument to get_lexer_for_filename):
...
    def colorize(filename, contents):
        # Don't want any errors.
        try:
            lexer = lexers.get_lexer_for_filename(filename)
            formatter = formatters.TerminalFormatter()
            return highlight(contents, lexer, formatter)
...
which include option stripnl=True, which strips leading and trailing newlines.

This causes the python colorizer to strip the two leading newlines of async.c.

Fix this by initializing the lexer with stripnl=False.

Build and reg-tested on x86_64-linux.

gdb/ChangeLog:

2020-04-10  Tom de Vries  <tdevries@suse.de>

	PR cli/25808
	* python/lib/gdb/__init__.py: Initialize lexer with stripnl=False.

gdb/testsuite/ChangeLog:

2020-04-10  Tom de Vries  <tdevries@suse.de>

	PR cli/25808
	* gdb.base/style.c: Add leading newlines.
	* gdb.base/style.exp: Use gdb_get_line_number to get specific lines.
	Check listing of main's one-line body.

gdb/python/lib/gdb/__init__.py

@@ -216,7 +216,7 @@ try:
     def colorize(filename, contents):
         # Don't want any errors.
         try:
-            lexer = lexers.get_lexer_for_filename(filename)
+            lexer = lexers.get_lexer_for_filename(filename, stripnl=False)
             formatter = formatters.TerminalFormatter()
             return highlight(contents, lexer, formatter)
         except: