mergetool(vimdiff): allow paths to contain spaces again

In 0041797449d (vimdiff: new implementation with layout support, 2022-03-30), we introduced a completely new implementation of the `vimdiff` backend for `git mergetool`. In this implementation, we no longer call `vim` directly but we accumulate in the variable `FINAL_CMD` an arbitrary number of commands for `vim` to execute, which necessitates the use of `eval` to split the commands properly into multiple command-line arguments. That same `eval` command also needs to pass the paths to `vim`, and while it looks as if they are quoted correctly, that quoting only reaches the `eval` instruction and is lost after that, therefore paths that contain whitespace characters (or other characters that are interpreted by the POSIX shell) are handled incorrectly. This is a simple reproducer: git init -b main bam-merge-fail cd bam-merge-fail echo a>"a file.txt" git add "a file.txt" git commit -m "added 'a file.txt'" echo b>"a file.txt" git add "a file.txt" git commit -m "diverged b 'a file.txt'" git checkout -b c HEAD~ echo c>"a file.txt" git add "a file.txt" git commit -m "diverged c 'a file.txt'" git checkout main git merge c git mergetool --tool=vimdiff With Git v2.37.0/v2.37.1, this will open 7 buffers, not four, and not display the correct contents at all. To fix this, let's not expand the variables containing the path parameters before passing them to the `eval` command, but let that command expand the variables instead. This fixes https://github.com/git-for-windows/git/issues/3945 Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>