Configuring vimdiff splits with specific statusline / line-numbers

I'm trying to set up git mergetool to spawn vimdiff with the following splits (and a configuration customized to each split):

~~Splits from the following buffers $LOCAL, $BASE, $REMOTE, & $MERGED (horizontal)~~ [done]
Show line-numbers per split [Todo]
Display only the name of the buffer variable in these three splits: $LOCAL, $BASE, $REMOTE. [Todo]
~~The $MERGED split statusline should show the filename from %t [the $MERGED split is done]~~

I used this git configuration in my local repo:

git config merge.tool vimdiff
git config mergetool.vimdiff.cmd 'vim +"set number" +"set statusline=%t" -d -c "wincmd J" -O $MERGED $BASE $REMOTE $LOCAL'
git config merge.conflictstyle diff3
git config mergetool.prompt false

In the configuration above, a status line substring should be displayed per-split but I can't find the right way to build a regex to reduce the filename in the statusline down to just one of the three desired buffer variable names (i.e. "BASE", "REMOTE", or "LOCAL"). A regex isn't required, but it seems like a promising option.
Line numbers are only showing up in one split, but I want them in all four.

After much googling and many experiments, I still can't coerce vimdiff into showing the status line / numbers I want... I am not using a statusline plugin such as vim-powerline. How do I configure my git options to spawn vimdiff the way I need?

I included a screen capture of my existing git mergetool splits.

Solution

Adding line numbers for every split

In this command,vim +"set number" ... -O $MERGED $BASE $REMOTE $LOCAL, looks like vim intenally first creates the splits and then executes the set option. Refer: vim -c option.

There are 2 options to overcome this.

Use --cmd option: If we use, vim --cmd "set number" -O $MERGED $BASE $REMOTE $LOCAL, set number is applied before creating the splits. In fact, it is applied even before reading the .vimrc file. (Refer: vim --cmd option). It might cause concern if we have a overriding config in .vimrc like set nonumber.
Create splits manually: Instead of -O option, we can manually create the splits. This way after the -c option is applied, the splits will be created. We will send the file parameters in the reverse order as below:
```
vim +"set number" +"vs $LOCAL" +"vs $BASE" +"vs $MERGED" +"wincmd J" $REMOTE
```
But now we need another option (diff this) to enable diff in all splits, since -d only applies to the files passed directly in the command line.
```
vim +"set number" +"vs $LOCAL" +"vs $BASE" +"vs $MERGED" +"wincmd J" +"windo 
diffthis" $REMOTE
```

Modifying the status line

Rather than regex, using static strings looks a more reliable option. We can leverage vim setlocal options to acheive this. So the total command will be

vim +"set number" +"setlocal statusline=REMOTE" +"vs $LOCAL" +"setlocal statusline=LOCAL" +"vs $BASE" +"setlocal statusline=BASE" +"vs $MERGED" +"wincmd J" +"windo diffthis" $REMOTE

Git mergetool config:

git config merge.tool vimdiff
git config mergetool.vimdiff.cmd 'vim +"set number" +"setlocal statusline=REMOTE" +"vs $LOCAL" +"setlocal statusline=LOCAL" +"vs $BASE" +"setlocal statusline=BASE" +"vs $MERGED" +"wincmd J" +"windo diffthis" $REMOTE'
git config merge.conflictstyle diff3
git config mergetool.prompt false

git mergetool output:

Update 1

If it is ok to retain the existing status line or do a simple append of the buffer number of the split, then -S vim option can be used to greatly simplify the command.

git config mergetool.vimdiff.cmd 'vim -d -S "~/.git_merge_vimrc" -O $MERGED $BASE $REMOTE $LOCAL

.git_merge_vimrc contents:

set statusline=%t-Split#%n
wincmd J
windo set number

Note: vim scripts can be used in the source file to print complex status line if that is required.