fix(NeogitOrg#23): keep commit message around if commit fails

Since the commit can fail for various reasons, we keep the commit
message file after the user closes the buffer and only remove it after
the commit was successful. Upon opening the commit buffer, we read the
commit file's contents, if it exists, and present it as the suggested
commit message

lua/neogit/async/uv.lua

@@ -0,0 +1,23 @@
+local uv = vim.loop
+local a = require('neogit.async')
+
+local wrapper = setmetatable({}, {
+  __index = function (tbl, action)
+    if uv[action] then
+      return a.wrap(uv[action])
+    end
+
+    return nil
+  end
+})
+
+wrapper.read_file = a.sync(function (file)
+  local err, fd = a.wait(wrapper.fs_open(file, 'r', 438))
+  if err then return nil end
+  local _, stat = a.wait(wrapper.fs_fstat(fd))
+  local _, data = a.wait(wrapper.fs_read(fd, stat.size, 0))
+  a.wait(wrapper.fs_close(fd))
+  return data
+end)
+
+return wrapper

lua/neogit/popups/commit.lua

@@ -2,6 +2,8 @@ local popup = require("neogit.lib.popup")
 local cli = require("neogit.lib.git.cli")
 local Buffer = require("neogit.lib.buffer")
 local a = require('neogit.async')
+local uv = require('neogit.async.uv')
+local split = require('neogit.lib.util').split
 
 local get_commit_message = a.wrap(function (content, cb)
   Buffer.create {
@@ -31,7 +33,7 @@ end)
 local prompt_commit_message = a.sync(function (msg)
   local output = {}
 
-  if msg then
+  if msg and #msg > 0 then
     for _, line in ipairs(msg) do
       table.insert(output, line)
     end
@@ -123,9 +125,12 @@ local function create()
           description = "Commit",
           callback = function(popup)
             a.dispatch(function ()
-              a.wait(prompt_commit_message(nil))
+              local data = a.wait(uv.read_file('.git/COMMIT_EDITMSG'))
+              local old_content = split(data or '', '\n')
+              a.wait(prompt_commit_message(old_content))
               local _, code = a.wait(cli.commit.commit_message_file('.git/COMMIT_EDITMSG').args(unpack(popup.get_arguments())).call())
               if code == 0 then
+                a.wait(uv.fs_unlink('.git/COMMIT_EDITMSG'))
                 __NeogitStatusRefresh(true)
               end
             end)