[rubygems/rubygems] Feature: accept pull request URLs as github source

Very often github source is used to temporarily use a modified gem
while a PR upstream is being reviewed.

So for instance https://github.com/ruby/bigdecimal/pull/211 will look like:

```ruby
gem "bigdecimal", github: "casperisfine/bigdecimal", branch: "git-gem" # https://github.com/ruby/bigdecimal/pull/200
```

It's annoying because you have to fiddle with the branch name, which is copied as `casperisfine:git-gem`, etc etc.

If I could simply use the PR URL like this:

```
gem "bigdecimal", github: "https://github.com/ruby/bigdecimal/pull/211"
```

It would make a very common task for me so much simpler.

https://github.com/rubygems/rubygems/commit/517c527751

1 files changed, 18 insertions, 3 deletions

diff --git a/lib/bundler/dsl.rb b/lib/bundler/dsl.rb
index ed7b3e2d6b..1108fc3b78 100644
--- a/lib/bundler/dsl.rb
+++ b/lib/bundler/dsl.rb
@@ -18,6 +18,8 @@ module Bundler
     VALID_KEYS = %w[group groups git path glob name branch ref tag require submodules
                     platform platforms type source install_if gemfile].freeze
 
+    GITHUB_PULL_REQUEST_URL = %r{\Ahttps://github\.com/([A-Za-z0-9_\-\.]+/[A-Za-z0-9_\-\.]+)/pull/(\d+)\z}.freeze
+
     attr_reader :gemspecs
     attr_accessor :dependencies
 
@@ -278,8 +280,17 @@ module Bundler
         warn_deprecated_git_source(:github, <<-'RUBY'.strip, 'Change any "reponame" :github sources to "username/reponame".')
 "https://github.com/#{repo_name}.git"
         RUBY
-        repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
-        "https://github.com/#{repo_name}.git"
+        if repo_name =~ GITHUB_PULL_REQUEST_URL
+          {
+            "git" => "https://github.com/#{$1}.git",
+            "branch" => "refs/pull/#{$2}/head",
+            "ref" => nil,
+            "tag" => nil,
+          }
+        else
+          repo_name = "#{repo_name}/#{repo_name}" unless repo_name.include?("/")
+          "https://github.com/#{repo_name}.git"
+        end
       end
 
       git_source(:gist) do |repo_name|
@@ -365,7 +376,11 @@ repo_name ||= user_name
 
       git_name = (git_names & opts.keys).last
       if @git_sources[git_name]
-        opts["git"] = @git_sources[git_name].call(opts[git_name])
+        git_opts = @git_sources[git_name].call(opts[git_name])
+        git_opts = { "git" => git_opts } if git_opts.is_a?(String)
+        opts.merge!(git_opts) do |key, _gemfile_value, _git_source_value|
+          raise GemfileError, %(The :#{key} option can't be used with `#{git_name}: #{opts[git_name].inspect}`)
+        end
       end
 
       %w[git path].each do |type|