Fix: make C-c C-o recursively locate a .md file

[srid]

When configuring wiki-links to allow whitespace in filenames, there is a bug in C-c C-o not really locating files recursively.

Description

When using:

(use-package markdown-mode
  :config
  (setq markdown-enable-wiki-links t)
  (setq markdown-wiki-link-search-subdirectories t)
  (setq markdown-link-space-sub-char " "))

C-c C-o will bail out if the target exists in sub-directories.

Debugger entered--Lisp error: (wrong-type-argument arrayp nil)
  substring(nil 0 0)
  replace-regexp-in-string("[[:space:]\n]" " " nil)
  markdown-convert-wiki-link-to-filename(nil)
  markdown-follow-wiki-link(nil nil)
  markdown-follow-wiki-link-at-point(nil)
  markdown-follow-thing-at-point(nil)
  funcall-interactively(markdown-follow-thing-at-point nil)
  call-interactively(markdown-follow-thing-at-point nil nil)
  command-execute(markdown-follow-thing-at-point)

Using save-match-data fixes that.

Related Issue

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves an existing feature)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• I have read the CONTRIBUTING.md document.
• I have updated the documentation in the README.md file if necessary.
• I have added an entry to CHANGES.md.
• I have added tests to cover my changes.
• All new and existing tests passed (using make test).

[syohex]

Could you show us how to reproduce your problem in detail(with sample markdown text etc) ?

[srid]

To reproduce,

git clone https://github.com/srid/neuron.git
cd doc 
emacs Guide.md 

Using the emacs configuration in issue description, try navigating to most of the links in Guide.md.

[syohex]

I have added some fixes to this PR and sent PR #591. Please check it.

[srid]

Confirmed to be working, and commented in detail in the other PR. Closing this one.