vim:keys: Fix commit reference when at EOL

Fix the commit reference mapping `<leader>grc` in the case that nothing
follows the commit hash on the line. In that case the join would add a
trailing space.

`gJ` is not suitable as that would break in the other case that text
follows. When splitting the line behind the commit hash, the space in
front of whatever follows is deleted. Thus when joining everything
together at the end, there would be no space between the closing
parentheses and the rest.

Using the `system()` was the only solution I found to insert the subject
directly behind the hash without destroying the formatting and the text
behind it.

For this to work the trailing newline had to be cut off of git's output.

.config/vim/vimrc.d/40-keys.vim

@@ -113,7 +113,9 @@ nnoremap <expr> gp '`[' . strpart(getregtype(), 0, 1) . '`]'
 " Git bindings
 " Insert a commit's subject behind the SHA1 that the cursor is currently on.
 " Mnemonic: "git reference commit"
-nmap <leader>grc yiwea<CR><Esc>k:r!git show -s --format='("\%s")' <C-R>0<CR>kJJ
+" NOTE: This uses `system` and not `:r!` to insert the text directly at the
+"       cursor. `subject[:-2]` cuts off the trailing newline.
+nmap <leader>grc :let subject=system('git show -s --format="(\"%s\")" <C-R><C-W>')<CR>ea <C-R>=subject[:-2]<CR><Esc>
 if exists('g:loaded_fugitive')
 	nmap <leader>gc :G commit<CR>