Tag renaming

[anjiro]

This PR adds functionality to rename tags. Some highlights:

tags.vim:

• wiki#tags#rename(old_tag, new_tag='', rename_to_existing=0)
  The main renaming function. The rename_to_existing parameter forces a tag to be renamed even if the new name already exists (implemented primarily for non-interactive testing).
• wiki#tags#rename_ask(old_tag='', new_tag='', ...)
  The user-facing rename function, called by the WikiTagRename command. Prompts for one or both parameters.
• wiki#tags#get_tag_names(...)
  Returns a list of tag names from the cache. Used for completion by WikiTagRename.
• g:wiki#tags#default_parser.make(taglist, curline='')
  Required to construct a new buffer line containing the updated tag name.

buffer.vim:

• Command WikiTagRename
• Default mapping: <leader>wsn → <plug>(wiki-tag-rename)

Tests:

• Added test cases for renaming tags to test-tags/test-defaults.vim and test-tags/test-custom-yaml.vim

There is some overlap in functionality between tags.vim:394-407,420-424 and page.vim:118-132,137-141, but I wasn’t sure if it was worth pulling out into a third file.

[lervag]

Wow, this is quite impressive. I do have some comments. I'll probably make minor adjustments myself, so feel free to ignore the trivial comments if you find me too pedantic. :p

In general, it seems this is pretty much ready for merging, but there a couple of important comments to discuss:

• Version requirement due to optional function args.
• Use customlist instead of custom for completion and adjust the get_tag_names output to list instead of string?

I think the overlap between page.vim and tags.vim is OK for now. I agree this could probably be generalised, but let's be pragmatic.

[lervag]

Huh, I was not aware of the default value syntax. It seems it was introduced quite recently? I like it, but it does set a requirement to the users to use Vim 8.1 and/or neovim 0.5. Is that OK? I never did specify a version requirement...

[anjiro]

I’m a Python programmer so default args like that make me happy. It would be simple to change it to use … instead, but not as nice looking. I have no sense of what versions people are using currently; it seems like one would naturally use an up-to-date Vim for one’s personal wiki? But… 🤷

[lervag]

I'm also quite used to Python, and I tend to like this syntax myself. I would not mind to accept this, but we should add a comment in the README and docs about version requirement.

[anjiro]

I will attack any pedantry-related changes and anything else when I have some time, possibly during the weekend; or of course feel free to do it for me 😉.

[lervag]

Thanks! I've merged this now. Very nice work, I appreciate the contribution!

[lervag]

FYI: I've reverted the modern optional arg syntax due to #194.

[anjiro]

Yes, makes sense, the nicer syntax probably isn't worth the compatibility issues.