[Rakefile] Generate man/index in man:build

Results in the following:

```
Gemfile(5)            gemfile.5
bundle(1)             bundle.1
bundle-add(1)         bundle-add.1
bundle-binstubs(1)    bundle-binstubs.1
bundle-check(1)       bundle-check.1
bundle-clean(1)       bundle-clean.1
bundle-config(1)      bundle-config.1
bundle-exec(1)        bundle-exec.1
bundle-gem(1)         bundle-gem.1
bundle-info(1)        bundle-info.1
bundle-init(1)        bundle-init.1
bundle-inject(1)      bundle-inject.1
bundle-install(1)     bundle-install.1
bundle-lock(1)        bundle-lock.1
bundle-open(1)        bundle-open.1
bundle-outdated(1)    bundle-outdated.1
bundle-package(1)     bundle-package.1
bundle-platform(1)    bundle-platform.1
bundle-pristine(1)    bundle-pristine.1
bundle-show(1)        bundle-show.1
bundle-update(1)      bundle-update.1
bundle-viz(1)         bundle-viz.1
```

2 files changed, 19 insertions, 17 deletions

diff --git a/Rakefile b/Rakefile
index d9ffc70059..bb37a9666f 100644
--- a/Rakefile
+++ b/Rakefile
@@ -240,12 +240,15 @@ begin
   namespace :man do
     directory "man"
 
+    index = []
     sources = Dir["man/*.ronn"].map {|f| File.basename(f, ".ronn") }
     sources.map do |basename|
       ronn = "man/#{basename}.ronn"
-      manual_section = ".1" unless basename =~ /.*(\d+)\Z/
+      manual_section = ".1" unless basename =~ /\.(\d+)\Z/
       roff = "man/#{basename}#{manual_section}"
 
+      index << [ronn, File.basename(roff)]
+
       file roff => ["man", ronn] do
         sh "#{Gem.ruby} -S ronn --roff --pipe #{ronn} > #{roff}"
       end
@@ -257,9 +260,23 @@ begin
       task :build_all_pages => "#{roff}.txt"
     end
 
+    file "index.txt" do
+      index.map! do |(ronn, roff)|
+        [File.read(ronn).split(" ").first, roff]
+      end
+      index = index.sort_by(&:first)
+      justification = index.map {|(n, _f)| n.length }.max + 4
+      File.open("man/index.txt", "w") do |f|
+        index.each do |name, filename|
+          f << name.ljust(justification) << filename << "\n"
+        end
+      end
+    end
+    task :build_all_pages => "index.txt"
+
     task :clean do
       leftovers = Dir["man/*"].reject do |f|
-        File.extname(f) == ".ronn" || f == "man/index.txt"
+        File.extname(f) == ".ronn"
       end
       rm leftovers if leftovers.any?
     end
diff --git a/man/index.txt b/man/index.txt
deleted file mode 100644
index e3b38c46db..0000000000
--- a/man/index.txt
+++ /dev/null
@@ -1,15 +0,0 @@
-Gemfile(5)         gemfile.5
-bundle-install     bundle-install.1
-bundle-update      bundle-update.1
-bundle-package     bundle-package.1
-bundle-exec        bundle-exec.1
-bundle-config      bundle-config.1
-bundle-platform    bundle-platform.1
-bundle-gem         bundle-gem.1
-bundle-clean       bundle-clean.1
-bundle-check       bundle-check.1
-bundle-init        bundle-init.1
-bundle-inject      bundle-inject.1
-bundle-open        bundle-open.1
-bundle-show        bundle-show.1
-bundle-viz         bundle-viz.1