Bug 1506173

Description of problem:
S2I Build fails with "Failed to fetch source for build" when git repository contains a broken link on OCP 3.5.

Version-Release number of selected component (if applicable):
3.5

How reproducible:
on a git projet create a link (ln -s) on an existing file f, delete the file f 
Or use the sample project at: https://github.com/erreff/carsten

Steps to Reproduce:
Create a project with a broken symbolic link. As an example you can use this project against a PHP template: https://github.com/erreff/carsten

Actual results:
Cloning "https://github.com/erreff/carsten" ...
	Commit:	352d2b6c173d63b7a5eb859ad6a1924a4cd506fe (add bad link)
	Author:	root <root.extra.laposte.fr>
	Date:	Tue Oct 24 18:07:29 2017 +0200
error: build error: open /tmp/s2i-build430207290/upload/src/contextdir/badlink: no such file or directory


Expected results:
In OCP 3.4, the build process is not stopped in case a symbolic link is broken.

Cloning "https://github.com/erreff/carsten" ...
	Commit:	352d2b6c173d63b7a5eb859ad6a1924a4cd506fe (add bad link)
	Author:	root <root.extra.laposte.fr>
	Date:	Tue Oct 24 18:07:29 2017 +0200

---> Installing application source...
chgrp: cannot dereference './contextdir/badlink': No such file or directory
chmod: cannot operate on dangling symlink './contextdir/badlink'


Pushing image 172.30.1.1:5000/demo/demophp:latest ...
Pushed 0/5 layers, 20% complete
Pushed 1/5 layers, 21% complete
Pushed 2/5 layers, 41% complete
Pushed 3/5 layers, 71% complete
Pushed 4/5 layers, 100% complete
Pushed 5/5 layers, 100% complete
Push successful

$ oc get pods
NAME              READY     STATUS      RESTARTS   AGE
demophp-1-0lngh   1/1       Running     0          3m
demophp-1-build   0/1       Completed   0          4m


Additional info: