completion-ignored-extensions: match full file names

[David Reitter]

After much searching, I finally figured out why completion did not work for in 
a specific, often-used case.

I gave several git repositories a ".git" ending in order to comply with the 
requirements of a program that comes with git (GitX).  One repository in the 
same directory does not have the ".git" extension.

Completion always choose the one directory without the extension, because 
`completion-ignored-extensions' contains ".git/".  So, matching ".git/" is a 
bit too general - I'd rather match something like "^\.git" in order to just 
exclude the internal ".git" directories.  Is that possible, and if not, would 
be it easy to realize?