PageLayoutHelper#page_image uses Git to retrieve project images but this is not cached and rather slow

The method PageLayoutHelper#page_image is used to grab a logo from the Git repository to use in OpenGraph/Twitter meta tags. Unless a project has an avatar set via the web UI this method ends up calling Project#avatar_in_git which tries to figure out what the avatar is from the Git repository, it does this by checking if any of these files exists:

1. logo.png
2. logo.jpg
3. logo.gif

The Git operations are not cached meaning that for projects without an avatar every request ends up triggering a Git operation. Thinking of it we should probably do the following:

1. Cache the output of Project#avatar_in_git in Redis
2. Flush this cache whenever a commit is pushed that modifies any of the possible avatar images (but only when the commit actually contains any of these files in the diff stat)