[FIX][VIM] Set b:undo_ftplugin if not set

I hit the problem when loading a fugitive window, but this should be
done for all filetype plug-ins before trying to append to the variable.

vim/.vim/after/ftplugin/bash.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Use shfmt as ALE fixer for bash
 let b:ale_fixers=[ 'shfmt' ]
 let b:undo_ftplugin.='|unlet b:ale_fixers'

vim/.vim/after/ftplugin/c.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " More warnings and the usual version in flags for Clang
 let b:ale_c_clang_options='-Wall -Wextra -pedantic -std=c99'
 let b:undo_ftplugin.='|unlet b:ale_c_clang_options'

vim/.vim/after/ftplugin/cpp.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " More warnings and the usual version in flags for Clang
 let b:ale_cpp_clang_options='-Wall -Wextra -pedantic -std=c++17'
 let b:undo_ftplugin.='|unlet b:ale_cpp_clang_options'

vim/.vim/after/ftplugin/d.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Use my desired ALE fixer for D
 let b:ale_fixers=[ 'dfmt' ]
 let b:undo_ftplugin.='|unlet b:ale_fixers'

vim/.vim/after/ftplugin/fugitive.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Don't highlight trailing whitespace in fugitive windows
 let b:better_whitespace_enabled=0
 let b:undo_ftplugin.='|unlet b:better_whitespace_enabled'

vim/.vim/after/ftplugin/gitcommit.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Enable spell checking on commit messages
 setlocal spell
 let b:undo_ftplugin.='|setlocal spell<'

vim/.vim/after/ftplugin/json.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Use my desired ALE fixer for JSON
 let b:ale_fixers=[ 'jq' ]
 let b:undo_ftplugin.='|unlet b:ale_fixers'

vim/.vim/after/ftplugin/make.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Makefiles should use tabs to indent
 setlocal noexpandtab
 let b:undo_ftplugin.='|setlocal noexpandtab<'

vim/.vim/after/ftplugin/netrw.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Don't show Netrw in buffer list
 setlocal bufhidden=delete
 let b:undo_ftplugin='|setlocal bufhidden<'

vim/.vim/after/ftplugin/pandoc.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Let ALE know that I want Markdown linters
 let b:ale_linter_aliases=[ 'markdown' ]
 let b:undo_ftplugin.='|unlet b:ale_linter_aliases'

vim/.vim/after/ftplugin/python.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Use my desired ALE fixers for python
 let b:ale_fixers=[ 'black', 'isort' ]
 let b:undo_ftplugin.='|unlet b:ale_fixers'

vim/.vim/after/ftplugin/qf.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Use h/l to go to the previous/next non-empty quickfix or location list
 nnoremap <silent> <buffer> h :call quickfixed#older()<CR>
 let b:undo_ftplugin.='|nunmap <buffer> h'

vim/.vim/after/ftplugin/rust.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Check tests too
 let b:ale_rust_cargo_check_tests=1
 let b:undo_ftplugin='|unlet b:ale_rust_cargo_check_tests'

vim/.vim/after/ftplugin/sh.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Use shfmt as ALE fixer for sh
 let b:ale_fixers=[ 'shfmt' ]
 let b:undo_ftplugin.='|unlet b:ale_fixers'

vim/.vim/after/ftplugin/zsh.vim

@@ -1,3 +1,7 @@
+if !exists("b:undo_ftplugin")
+    let b:undo_ftplugin=''
+endif
+
 " Use shfmt as ALE fixer for zsh
 let b:ale_fixers=[ 'shfmt' ]
 let b:undo_ftplugin.='|unlet b:ale_fixers'