Instead of deciding provider status in eval_has_provider, move the
decision to the provider Vim scripts.
Previously, provider loading worked as follows:
1. eval_has_provider() verified provider availability by searching for
the provider#providername#Call function and cached this verificaion as a static
variable for some providers
2. providers short-circuited on loading to prevent the definition of the
Call function (with the exception of the node provider that did not)
This commit changes the expected interface between nvim and its
providers to facilitate provider reloading, by splitting the
verification of the provider from the availability of the Call function.
eval_has_provider() now checks for a provider#providername#enabled
variable. It is up to the provider script to set this to 0 or 1
accordingly. eval_call_provider() remains unchanged.
All providers hosting a Call function were updated to respect this.
The clipboard provider now has a Reload function to reload the
provider.
start_batch_changes() doesn't avoid invoking the clipboard
once-per-line, because the loop is actually in ex_echo(), which calls
redir_write() for each message. But we've already entered
start_batch_changes() by then, so that was never the problem.
redir_write at /home/vagrant/old.neovim/build/../src/nvim/message.c:2523
msg_puts_attr_len at /home/vagrant/old.neovim/build/../src/nvim/message.c:1600
msg_outtrans_len_attr at /home/vagrant/old.neovim/build/../src/nvim/message.c:1221
ex_echo at /home/vagrant/old.neovim/build/../src/nvim/eval.c:19433
do_one_cmd at /home/vagrant/old.neovim/build/../src/nvim/ex_docmd.c:2242
Trying to defer _explicit_ clipboard updates is difficult.
:redir @+ | silent echo system('cat foo') | redir END
is essentially equivalent to:
for l in readfile('foo')
let @+ .= l
endfor
We cannot make judgements about when to ignore a script's bad decisions.
start_batch_changes() only works around the case of clipboard=unnamed,
i.e. _implicit_ clipboard updates (`:g/foo/d`). Not explicit
assignment.
