Fix "Revspec not found" errors when viewing diffs in a forked project with submodules
## What does this MR do?
This MR fixes an error that occurs when viewing diffs in a forked project with submodules.
### Are there points in the code the reviewer needs to double check?
Testing this code was tricky. The only way this problem shows up is if the origin project does NOT have the submodule update commit. The introduction of gitlab-test-fork serves that purpose: it contains a submodule update not present in gitlab-test.
### Why was this MR needed?
A user would receive a 500 error when trying to view a merge request with a submodule update. #1413 has details on how to reproduce this issue.
### What are the relevant issue numbers / [Feature requests](http://feedback.gitlab.com/)?
#1413
See merge request !512
Update mentionable shared examples to be (a bit) more understandable
These shared examples were super confusing, and honestly still kind of are, but they should be a bit better now.
I encountered a bunch of headaches in these examples while refactoring ReferenceExtractor, so that's how I ended up changing it.
See merge request !1780
See http://schacon.github.io/git/git-init.html#_template_directory
Without this variable, any global git hooks a developer might have in
~/.git_template would be linked in the `.git/hooks` folder for every
test repository that gets checked out by TestEnv, and would cause
certain specs to fail due to pre-existing hook files.
This fixes one of the problems mentioned in gitlab-org/gitlab-ce#1162 and makes the behavior of the commit calendar as described in the profile.
"All email addresses will be used to identify your commits."
Before this it would fail because git hooks automatically prepend
things to the path, which can lead the wrong Ruby version to be called
in which dependencies are not installed.
To make sure that this is correct, the forked_merge_requests commented
out test that depends on this change was uncommented.
For that test to pass, it is also necessary to setup the mock server
on port 3001 under test_env.rb.
Robert Speicher
Dmitriy Zaporozhets
Douwe Maan
Dmitriy Zaporozhets
Stan Hu
Jeroen van Baarsen
Ciro Santilli
Valeriy Sizov
Vinnie Okada
Sullivan SENECHAL