NUL files written by Perforce vcs command when running vim from git-bash on Windows

# The problem

I am getting `NUL` files sprinkled here and there when using vim since updating vim-signify. Most annoyingly in the `.git/rebase-merge/` directory. A `NUL` file in there prevents git from removing the `rebase-merge` directory when rebasing is finished, locking git into a weird state. This is git's error message in that case: `error: could not remove '.git/rebase-merge'`.

# Note

I have a git repository inside a Perforce repository. We're working on some code separate from the larger Perforce code base. This is copied into a directory in the Perforce code base. This is weird and might muck things up.

# The cause

The reason for this odd behaviour of vim-signify seems to be the Perforce vcs command. Specifically when the logic is tripped and both cmd and bash things get combined into the command.

The offending command comes from autoload/sy/repo.vim:647:
` \ 'perforce': 'p4 info '. sy#util#shell_redirect('%n') . (has('win32') ? ' &&' : ' && env P4DIFF= P4COLORS=') .' p4 diff -du0 %f',`

Because I'm running on Windows `%n` is replaced by `NUL` in that command.

At the same time, because I'm running git, and thus vim, from git-bash, $SHELL is set to `/usr/bin/bash`. This sets vim's &shell to `C:\Program Files\Git\usr\bin\bash.exe`. This leads `wrap_cmd` to construct the command `"C:\Program Files\Git\usr\bin\bash.exe" -c " p4 info > NUL && p4 diff -du0 'E:\src\perforce_root_here\some\path\git_repo_here\.git\rebase-merge\git-rebase-todo' "`.

Now we're in trouble. Bash will pipe `p4 info` to the file `NUL` and not to something like `/dev/null` since bash doesn't understand that `NUL` should be something special.

# Hacky solution

I can solve this by replacing the Perforce vcs command with this

` \ 'perforce': 'p4 info '. '> /dev/null ' . (has('win32') ? ' &&' : ' && env P4DIFF= P4COLORS=') .' p4 diff -du0 %f',`

but that's not something one could commit. With that it works for my setup though.

Start in autoload/sy/repo.vim around line 647, where the Perforce VCS command uses sy#util#shell_redirect('%n'), and trace how wrap_cmd combines Vim's shell with the command. Reproduce the command from the issue on Windows using Git Bash and compare it with the native Windows-shell case. Done means Perforce checks no longer create NUL files while the existing command remains valid in other environments.

Written by the indexing model from the issue text.