extract: "Deeply nested" gettext calls aren't extracted

[oomsveta]

Overview

#488 enabled the python extractor to look at nested calls for strings to extract, but the way it does it ignores nested gettext calls when there's more than one level of nesting.

This is because scanning sources for nested calls is only done when call_stack is 0:

babel/babel/messages/extract.py

Lines 554 to 555 in f91754b

	elif funcname and call_stack == 0:
	nested = (tok == NAME and value in keywords)

call_stack starts at -1, and is incremented each time an opening parenthesis is encountered within a gettext call:

babel/babel/messages/extract.py

Lines 526 to 534 in f91754b

	elif tok == OP and value == '(':
	if in_def:
	# Avoid false positives for declarations such as:
	# def gettext(arg='message'):
	in_def = False
	continue
	if funcname:
	message_lineno = lineno
	call_stack += 1

In the case of a "deeply nested" call (a call with more than one level of nesting), the value of call_stack will be greater than 0, meaning that the part of the code that handles the nested call will never be reached, and the contents of that call won't be exported.

Steps to Reproduce

Copy-paste this to your python REPL:

from io import BytesIO
from babel.messages.extract import extract

file = b"""_(
    "dummy",
    nested_call(
        _("deeply nested call")
    )
)"""

list(extract("python", BytesIO(file)))  # -> [(4, 'dummy', [], None)]

Actual Results

[(4, 'dummy', [], None)]

"deeply nested call" is missing.
Note: the lineno of "dummy" is also wrong, see this other issue.

Expected Results

[(1, 'dummy', [], None), (4, 'deeply nested call', [], None)]