Vcs: Merge IVersionControl and VcsBasePlugin hierarchies

They were 1:1 in parallel, with quite a bit of function call
ping-pong inbetween, for code-sharing-by-inheritance. Merge
them by making VcsBasePlugin inherit IVersionControl and
merge the derived classes below.

Size of this patch is hard to avoid as all seven systems have to
move simultaneously. Non-necessary potential follow-up cleanup
have been left out on purpose.

Change-Id: Icb71e4182af3db21069cc637e7ae87ffa3829791
Reviewed-by: Orgad Shaneh <orgads@gmail.com>

src/plugins/git/git.pro

@@ -12,7 +12,6 @@ HEADERS += gitplugin.h \
     annotationhighlighter.h \
     gitsubmiteditorwidget.h \
     gitsubmiteditor.h \
-    gitversioncontrol.h \
     gitsettings.h \
     branchmodel.h \
     stashdialog.h \
@@ -36,7 +35,6 @@ SOURCES += gitplugin.cpp \
     annotationhighlighter.cpp \
     gitsubmiteditorwidget.cpp \
     gitsubmiteditor.cpp \
-    gitversioncontrol.cpp \
     gitsettings.cpp \
     branchmodel.cpp \
     stashdialog.cpp \