Adapt file link for GitLab 5.2

Fix the defective file link for GitLab browsers:

For older GitLab versions, there was a duplicate slash and for
newer versions the link is completely wrong. Fix both:

Old:   http://urlbase//0123456789...01234567/tree/file.name
<5.2:  http://urlbase/0123456789...01234567/tree/file.name
>=5.2: http://urlbase/blob/0123456789...01234567/file.name

Note: I do not know at which version of GitLab the new link
      format was introduced. This patch assumes 5.2.0, but it
      may be that older versions already need the new format.

Closes #JENKINS-18328
[FIXED JENKINS-7411]

Environment variables are in fact not used at all during expansions.
This commit fixes that. As discussed in the ticket, and as done
elsewhere, the standard convention is to use previous build's
environment to expand polling.

Also see JENKINS-19042
Update GitLab version number switch

The switch in commit 73249eeac is needed starting from GitLab 5.1.
Merge pull request #164 from olafmandel/JENKINS-18328

[FIXED JENKINS-18328] Defective file link for GitLab browsers
Use 'changeset' path for inspecting changesets -- not 'file'
changeset URL must not end with '/'
Merge pull request #167 from jmalm/patch-1

RhodeCode changeset URL
Update testGetChangeSetLinkGitChangeSet()
Merge pull request #168 from jmalm/patch-1

Update testGetChangeSetLinkGitChangeSet()
Removed possible NullPointerException during start
Merge pull request #169 from vralfy/master

Removed possible NullPointerException during start
git-client 1.2.0
Merge branch 'master' into refactoring

Conflicts:
	src/main/java/hudson/plugins/git/GitSCM.java
	src/test/java/hudson/plugins/git/GitSCMTest.java