Make adr files recognition stricter

[davebaol]

This fix makes adr list and adr link work properly when there are extraneous files along with adr files.

I just changed

find $adr_dir | grep -E "^$adr_dir/[0-9]+-[^/]*\\.md" | sort

to

find $adr_dir -maxdepth 1 -type f | grep -E "^.{${#adr_dir}}/[0-9]*[1-9][0-9]*-[^/]*\\.md$" | sort

Explanation:

• -maxdepth 1 exclude subfolders; this is faster if you have subfolders with a lot of files, for instance images linked by your ADRs
• -type f files only
• ^.{${#adr_dir}} skip as many chars as adr_dir length. This is more robust than matching the initial path with ^$adr_dir because adr_dir might contain special regex chars that should be escaped at runtime.
• [0-9]*[1-9][0-9]* any sequence of digits other than all zeros (0 is not a valid adr; for instance adr new starts from 1)
• \.md$ make sure the extension is exactly .md (for instance .md5 files are excluded)