`fish_prompt_pwd_full_dirs` variable gets reset to `1` every time a prompt appears

[jakubdabek]

The behavior is as follows (at current master):

<host> ~/l/k/f/build (master)> set -e fish_prompt_pwd_full_dirs
<host> ~/l/k/f/build (master)> set -S fish_prompt_pwd_full_dirs
<host> ~/l/k/f/build (master)> set -U fish_prompt_pwd_full_dirs 2
<host> ~/l/k/fish-shell/build (master)> set -S fish_prompt_pwd_full_dirs
$fish_prompt_pwd_full_dirs: set in universal scope, unexported, with 1 elements
$fish_prompt_pwd_full_dirs[1]: |1|
<host> ~/l/k/f/build (master)> set -U fish_prompt_pwd_full_dirs 2; set -S fish_prompt_pwd_full_dirs
$fish_prompt_pwd_full_dirs: set in universal scope, unexported, with 1 elements
$fish_prompt_pwd_full_dirs[1]: |2|
<host> ~/l/k/fish-shell/build (master)>
<host> ~/l/k/f/build (master)>

What's going on is that prompt_pwd incorrectly sets the global variable instead of the local one when using flags:

fish-shell/share/functions/prompt_pwd.fish

Lines 14 to 25 in deec78c

	set -ql _flag_d
	and set -l fish_prompt_pwd_dir_length $_flag_d
	set -q fish_prompt_pwd_dir_length
	or set -l fish_prompt_pwd_dir_length 1
	set -l fulldirs 0
	set -ql _flag_D
	and set fish_prompt_pwd_full_dirs $_flag_D
	set -q fish_prompt_pwd_full_dirs
	or set -l fish_prompt_pwd_full_dirs 1

See lines 15 and 22. The fix would be to set the local instead:

- and set fish_prompt_pwd_full_dirs $_flag_D
+ and set -l fish_prompt_pwd_full_dirs $_flag_D

Sidenote - the variable is changed every time a prompt appears, because prompt_pwd is called in fish_title:

fish-shell/share/functions/fish_title.fish

Line 11 in deec78c

echo -- $ssh (string sub -l 20 -- $argv[1]) (prompt_pwd -d 1 -D 1)