Enumerate reference link text when using markdown-insert-link

[mitchellroe]

When adding an implicit reference link via markdown-insert-link, the existing reference link names should be enumerated when entering the text.

Expected Behavior

Let's say my file looks like this (where the | is the cursor):

Here's another [website](http://foo.example.org).

See my | for more details.

[homepage]: https://www.example.org/homepage.html

1. Hit C-c C-l or M-x markdown-insert-link.
2. I am prompted for URL or [reference]:. I hit Tab and am offered a list of completions, which contains the following:
   1. [homepage]
   2. http://foo.example.org
   3. https://www.example.org/homepage.html
3. I leave the prompt blank and hit Enter.
4. I am prompted for Link text:. I hit Tab and am offered only one completion, homepage.
5. I type hom, hit Tab, and homepage is filled for me.
6. I hit Enter, and my file now looks like this:

Here's another [website](http://foo.example.org).

See my [homepage][]| for more details.

[homepage]: https://www.example.org/homepage.html

Actual Behavior

Starting with the same file as above:

Here's another [website](http://foo.example.org).

See my | for more details.

[homepage]: https://www.example.org/homepage.html

1. Hit C-c C-l or M-x markdown-insert-link.
2. I am prompted for URL or [reference]:. I hit Tab and am offered a list of completions, which contains the following:
   1. [homepage]
   2. http://foo.example.org
   3. https://www.example.org/homepage.html
3. I leave the prompt blank and hit Enter.
4. I am prompted for Link text:. I hit Tab.
5. A Tab character is inserted in the minibuffer.

Steps to Reproduce

I guess I got ahead of myself. See the Actual Behavior section.

Backtrace

No error is produced.

Software Versions

• Markdown Mode: markdown-mode, version 2.4-dev
• Emacs: GNU Emacs 26.2 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.30) of 2019-07-31
• OS: CentOS Linux release 7.7.1908

[syohex]

I have merged #447