Install git-core instead of git on RH systems

The git package pulls in heavy dependencies for just a few rarely
used commands. git-core should be enough (and already uses for Suse).

Change-Id: I96b71072c22c26b3b651466053b9e9561527cbe5

diskimage_builder/elements/source-repositories/pkg-map

@@ -3,6 +3,9 @@
     "gentoo": {
       "git": "dev-vcs/git"
     },
+    "redhat": {
+      "git": "git-core"
+    },
     "suse": {
       "git": "git-core"
     }