Changes in commit 5129e97285 (setup-user: switch to git-var to check if
username and e-mail are set, 2019-11-24, v3.17.0-rc1~379^2) now require
the `rev` and `cut` tools to be available. Revise the logic to use that
approach only when they are available, and otherwise fall back to the
previous more portable approach.
Co-Author: Brad King <brad.king@kitware.com>
In git, apart from setting username and e-mail in .gitconfig it's also
possible to set username in /etc/passwd and set e-mail using EMAIL
environment variable. The advantage of this method is that other
programs such as mutt or doxygen will pick up these settings up so
there is no need to set them separately in each program. Current way
of checking if username and e-mail are set using git config results in
failure if they are set using this method.
GitSetup upstream added an attribute to export `.gitattributes` so that
importing snapshots into the sources of other projects would bring along
the attributes. However, we don't want to export them from CMake. Drop
`.gitattributes` entries not relevant to CMake.
