[PATCH v2] mergetool(vimdiff): allow paths to contain spaces again

From: Johannes Schindelin <johannes.schindelin@gmx.de>

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>
---
    mergetool(vimdiff): allow paths to contain spaces again
    
    In https://github.com/git-for-windows/git/issues/3945, it was reported
    that as of v2.37.0, git mergetool --tool=vimdiff fails to handle paths
    containing spaces. Let's fix that.
    
    Changes since v1:
    
     * Using base_present=false instead of the misleading base_present=1
       (thanks Junio & Fernando!)

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1287%2Fdscho%2Ffix-vimdiff-with-spaces-in-paths-v2
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1287/dscho/fix-vimdiff-with-spaces-in-paths-v2
Pull-Request: https://github.com/gitgitgadget/git/pull/1287

Range-diff vs v1:

 1:  dde9c7cd2ea ! 1:  7de381b6913 mergetool(vimdiff): allow paths to contain spaces again
     @@ mergetools/vimdiff: run_unit_tests () {
      +		done
      +	}
      +
     -+	base_present=1
     ++	base_present=false
      +	LOCAL='lo cal'
      +	BASE='ba se'
      +	REMOTE="' '"

 mergetools/vimdiff | 39 +++++++++++++++++++++++++++++++++++----
 1 file changed, 35 insertions(+), 4 deletions(-)