bug#7461: 23.2; Gdb only does tab completion inside Emacs when prompt is (gdb)

[Rolando Pereira <finalyugi@sapo.pt>]

When running gdb inside emacs, either by doing M-x gdb or M-x gud-gdb,
if the prompt is set to something that isn't "(gdb)" (or similar, see
the examples below), pressing TAB doesn't do the completion of
commands, functions or variables.

Use the following steps to reproduce the error using emacs -Q:

1) Create a new C file, for example code.c, with the following
contents:

#include <stdio.h>

int main(void) {
	printf("Hello World\n");
	return 0;
}

2) Compile code using gcc -g code.c -o code

3) Open emacs -Q

4) Start debugging the file using M-x gdb

5) Prompt should be "(gdb) ". Try to type "b ma" and press TAB. It
should expand to "b main".

6) Change gdb's prompt to something else using "set prompt". I changed
it to "gdb> ".

7) Try to type "b ma" and press TAB. It should display on the
minibuffer the message "No completions of ma".


Prompts I've tried where TAB completion still worked:

* "(gdb)"
* "        (gdb)        " (the prefix whitespace is discarded by gdb,
  so this is the same as "(gdb)        ")
* "( gdb) "
* "(gdb)_______________________"
* "(GDB) "
* "( GDB) "

Prompts I've tried where TAB completion DIDN'T work:

* "(gdb )"
* "_(gdb)"
* "(gdb "
* "gdb) "
* "( gdb )"
* "" (removed prompt by typing "set prompt" in gdb)
* "gdb> "
* "gdb>"
* "gdb "
* "gdb"
* "<gdb "
* "<gdb"

Emacs version is: GNU Emacs 23.2.1 (i686-pc-linux-gnu, GTK+ Version
2.16.1)

GDB version is: GNU gdb (GDB) 7.2