Stop using 'repo' shorthand in internal variable names and functions

Problem/Motivation

Both internal and API code is using inconsistently repo and repository to refer to repository.

Proposed resolution

Use repository for all cases.

Remaining tasks

• Removed from simple strings and from documentation blocks.
• Changes on code variables.
• Changes on API strings(i.e. VersioncontrolRepositoryController::entityType), which affects a lot of code.
• Decide if we want to change reposync and repomgr. Which end up in many places too, since it's also a code name in API(i.e. ) and a ctools plugin type codename.
• Decide if we want to change the versioncontrol_handler_filter_repo_backend class name(not sure, but I guess that also affects code exported views).

User interface changes

None.

API changes

I think it worths to create a change notice about this, since it affects API.

Original report by sdboyer

When I was working heavily on the vcapi internals, I simply couldn't bring myself to write out the full word 'repository' ten thousand times. On reading & revisiting, though, it's kinda problematic - in particular, having the backend key on 'repo' instead of 'repository' for its various arrays of entity data is a bit of a gotcha. Like, I gotcha'd myself on it the other day when trying to implement hook_query_alter(), because I was expecting 'repository' and it was actually 'repo'. All of our other entities use the full word, repositories should too.