[OS-BUILD PATCHv4 0/2] redhat/Makefile, Makefile.common: add GIT macro

[GitLab Bridge on behalf of bcrocker]

From: bcrocker on gitlab.com

In redhat/Makefile and Makefile.common, add a GIT macro:

GIT ?= git

and replace literal occurrences of 'git' with $(GIT).
This change enables us to override 'git' with, e.g., some
arbitrary shell script that prints additional information
and/or does additional processing before and/or after (or
even instead of) invoking /usr/bin/git.