According to msdn documentation last backslash(es) of quoted argument
in a win32 cmdline need to be escaped, since they are
directly preceding quote in the resulting string:
https://docs.microsoft.com/en-us/cpp/c-language/parsing-c-command-line-arguments

Glib <=2.58.0 passed children arguments like C:\Program Files\
without escaping last backslash(es).
So it had been passed as "C:\Program Files\"
windows command line parsing treated this as escaped quote,
and later text was treated as argument continuation instead of separate
arguments.

Existing implementation wasn't easily adoptable to fix this problem,
so escaping logic was rewritten.
Since the resulting length need to be increased due to extra escaping
it was rewritten too. Now the calculated length assumes that all
escapable chars would be escaped in a resulting string,
so the length may be a bit bigger than actually needed,
since backslashes not preceding quotes are not escaped.

This fixes the glib/tests/spawn-singlethread.c test
(which introduced testing for special chars to make this problem
testable).
The problem itself was found during investigations about fixing
related #1566

The logic is duplicated in protect_argv_string() and protect_wargv() funcs.
However there is no single obvious way to get rid of duplication -
!419 (comment 371483)

So by now adding a note referencing protect_wargv from protect_argv_string,
the other direction is already referenced.