Move git specific stuff out of diff editor plugin

Move it to the git plugin.

Change-Id: I8151573ed50df70776f7ebf0475dd41fb84fae83
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/diffeditor/diffeditor.pro

@@ -1,7 +1,9 @@
 DEFINES += DIFFEDITOR_LIBRARY
 include(../../qtcreatorplugin.pri)
 
-HEADERS += diffeditor_global.h \
+HEADERS += \
+        descriptionwidgetwatcher.h \
+        diffeditor_global.h \
         diffeditor.h \
         diffeditorconstants.h \
         diffeditorcontroller.h \
@@ -17,7 +19,9 @@ HEADERS += diffeditor_global.h \
         unifieddiffeditorwidget.h \
         diffeditoricons.h
 
-SOURCES += diffeditor.cpp \
+SOURCES += \
+        descriptionwidgetwatcher.cpp \
+        diffeditor.cpp \
         diffeditorcontroller.cpp \
         diffeditordocument.cpp \
         diffeditorfactory.cpp \