Features:
    c7d84c5550a1 PR #4980 Full `:ruby` support!
    c74ce334f2f7 PR #4624 timers: timer_start(), timer_stop()
    b8e6f04e6952 PR #5205 `:CheckHealth` command

    47a15d025617 PR #4865 file: Add buffered reading and writing
        *Much* faster shada file reading (important for startup time).

    71b3e20d0fba PR #4723 jobstart() learned 'rpc'
        jobs and RPC channel IDs share the same "namespace".
        jobstart() can starts RPC channels, which allows scripts to handle
        'stderr' on a RPC channel, like a typical non-RPC job.
        jobpid()/jobstop() work on RPC "jobs".
        Deprecates rpcstart().

    4dc4efc36fad PR #4449 man.vim rewrite
        `:Man` command is enabled by default.
        New features: completion, window handling, better parsing, and more.

    8a4e5b4bc237 PR #4697 capture() function (renamed to execute())
        Supports nesting, including nested :redir.

    ae6db26b0956 PR #5050 'rplugin manifest: default to XDG dir'
    a1682281f427 PR #5214 Restore ":browse oldfiles".
    1f7304b84681 Better handling of mouse-clicks on concealed chars.
    5ea4d58a1b1d PR #5026 terminal: Ensure b:term_title always has a value
    c00231078790 tui: Assume 256 colors in most cases.
    a2ecbc2cc093 PR #4929 Always resize the :terminal
    a59330d6fc99 PR #4925 api_info()
    a160590e4034 PR #4813 allow setting cwd in jobstart(), termopen()
    74f64601817a PR #4633: support "special" highlight (undercurl)
    5a5ef1c2227f PR #3450 mouse: Implement horizontal scroll.

Windows support:
    All PRs now build on Appveyor targeting win32 and win64!
    Numerous fixes!

Fixes:
    e9061117a5b8 PR #4646 Prevent data loss for process output streams
    7fa1baf44e78 PR #4798 'process.c: Fix block in teardown'
    c10fe010f165 Prevent endless loop in printdigraph(). (#5215)
    add41dca98b7 PR #5192 timers: Avoid crash after processing events
    006f9c0c9c96 PR #5195 Set the default value for 'packpath'
    6da7d6890cc6 PR #5025 Restore double click
    d622e9c41635 readfile(): Less-disruptive readonly check.
        Fixes an issue where nvim unnecessarily "touched" open files.
    fe6ec757257d PR #4964 Handle very long $XDG_DATA_DIRS.
    895f712df8af option: Do not expand options in XDG vars.
    1d8a07615714 server_init: Handle server_address_new() failure.
    be531aba777a PR #5042 Fix v:register for clipboard=unnamed,unnamedplus
    204f557a11e2 PR #4984 'Trigger TabNewEntered with <CTRL-W>T'
    1e93e24f5e6e PR #4851 synIDattr(): Return RRGGBB value for `fg#`.

Changes:
    acc5d08b371c PR #4690 'termguicolors' option enables "true color".
        NVIM_TUI_ENABLE_TRUE_COLOR is now ignored.

:silent does not silence this message, even :redir does not consume it.
But execute() _does_ consume it, which interferes with the current
implementation of health.vim. It's prudent to avoid it in any case, even
if the implementation of health.vim changes in the future.

We can add this later if it is proven necessary, but it should not be
because:

1. User can run a subset of checkers via `:CheckHealth plugin1, ...,`
2. Healthcheck is a very rare operation. Optimizing it is not worth the
   code/API complexity.

To healthcheck the "foo" plugin:
    :CheckHealth foo
To healthcheck the "foo" and "bar" plugins:
    :CheckHealth foo bar
To run all auto-discovered healthchecks:
    :CheckHealth

- Overlay markdown syntax/filetype, don't invent new filetypes/syntaxes.
- migrate s:check_ruby()
- s:indent_after_line1
- Less-verbose output

- Use execute() instead of redir
- Fixed logic on suboptimal pyenv/virtualenv checks.
- Move system calls from strings to lists. Fixes #5218
- Add highlighting
- Automatically discover health checkers
- Add tests

Helped-by: Shougo Matsushita <Shougo.Matsu@gmail.com>
Helped-by: Tommy Allen <tommy@esdf.io>

Closes #4932

vim-patch:7.4.1700,7.4.2219

* sub_joining_lines: Optimization for :%s/\n//
* sub_grow_buf: Allocation of buffer to contain replacement text
* sub_parse_flags: Parse {flags} from :s command into subflags_T

Although this doesn't reduce do_sub's size enough to satisfy lint, it
covers the more straightforward pieces.