mcf: Fix detecting wrong local or remote branches

:Release Notes:
Search for exact match, not just branch ending with the same substring.

:Detailed Notes:
It was incorrectly changing tracking branch, e.g.:
qt5-5.0.2 -> before-qt5-5.0.2
master -> jansa/master
There is hardcoded "origin" in couple places already, so it doesn't
make it worse to enforce origin/$branchname here as well (at least
following "git pull origin" won't fail to fetch right revisions, if the
tracking branch is really in some other remote than "origin").

:Testing Performed:
Only build tested.

:QA Notes:
No changes to image.

:Issues Addressed:
[GF-60135] mcf sometimes sets wrong remote branch for tracking

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: Ie3861e905682a9b6620b0ebf9593181ea51d081c
Reviewed-on: https://g2g.palm.com/4769
Reviewed-by: DCO Verification
Reviewed-by: Martin Jansa <Martin.Jansa@lge.com>
Tested-by: Martin Jansa <Martin.Jansa@lge.com>
Build: Martin Jansa <Martin.Jansa@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>