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