reintegrate: add vim syntax highlighting

This will still involve some hoop-jumping to install, since Homebrew
isn't really supposed to be used to add local Vim extensions. AFAICT,
Vim doesn't have any patches under Homebrew that tell it to look for
system-wide configurations under /usr/local/etc, so we can't just
install things there (unlike bash_completion or zsh_completion).

Instead, the idea at this point is to basically take the same approach
as https://github.com/junegunn/fzf, where the brew formula has a caveat
telling you how to add a custom path to Vim's runtimepath variable. All
that means for this repo is that we have to make sure to bundle the vim
syntax files in the release tarball. So besides the vim source itself,
just have to amend the (ghetto) build script a little bit.

Fixes #2.

build

@@ -1,5 +1,15 @@
 #!/usr/bin/env bash
 
+FILES=(
+  LICENSE
+  README.md
+  RATIONALE.md
+  completions
+  libexec
+  man
+  vim
+)
+
 VERSION="$(git tag --sort=version:refname | tail -1 | sed 's/^v//')"
 test -n "$VERSION" || VERSION="0.0.1"
 
@@ -30,9 +40,7 @@ echo >&2 "Generating $build..."
 if ! test -d builds; then
   mkdir builds
 fi
-tar -cvzf "$build" LICENSE README.md RATIONALE.md completions libexec man 2> >(
-  sed 's/^/     /'
-)
+tar -cvzf "$build" "${FILES[@]}" 2> >(sed 's/^/     /')
 
 echo >&2
 echo >&2 "Generated $build"

vim/ftdetect/git-topics-reintegrate.vim

@@ -0,0 +1,2 @@
+autocmd BufNewFile,BufRead *.git/git-topics-reintegrate/todo.new
+      \ setfiletype git-topics-reintegrate

vim/syntax/git-topics-reintegrate.vim

@@ -0,0 +1,25 @@
+" Vim syntax file
+" Language:   git topics reintegrate
+" Maintainer: Alex Vondrak (https://github.com/ajvondrak)
+" Repository: https://github.com/ajvondrak/git-topics
+"
+" Based on the git rebase --interactive highlighting by Tim Pope:
+" https://github.com/vim/vim/blob/89bcfda/runtime/syntax/gitrebase.vim
+
+if exists("b:current_syntax")
+  finish
+endif
+
+syn case match
+
+syn match reintegratePick    "\v^p%(ick)=>" nextgroup=reintegrateTopic skipwhite
+syn match reintegrateDrop    "\v^d%(rop)=>" nextgroup=reintegrateTopic skipwhite
+syn match reintegrateTopic   ".*"           contained
+syn match reintegrateComment "^#.*"
+
+hi def link reintegratePick    Statement
+hi def link reintegrateDrop    Comment
+hi def link reintegrateTopic   Identifier
+hi def link reintegrateComment Comment
+
+let b:current_syntax = "git-topics-reintegrate"