install/devshell: Introduce git intercept script due to fakeroot issues

In a devshell, recent versions of git will complain if the repo is owned
by someone other than the current UID - consider this example:

 ------
  bitbake -c devshell linux-yocto

  [...]

  kernel-source#git branch
  fatal: unsafe repository ('/home/paul/poky/build-qemuarm64/tmp/work-shared/qemuarm64/kernel-source' is owned by someone else)
  To add an exception for this directory, call:

        git config --global --add safe.directory /home/paul/poky/build-qemuarm64/tmp/work-shared/qemuarm64/kernel-source
  kernel-source#
 ------

Of course the devshell has UID zero and the "real" UID is for "paul" in
this case.  And so recent git versions complain.

As the whole purpose of the devshell is to invoke a shell where development
can take place, having a non-functional git is clearly unacceptable.

Richard suggested we could use PSEUDO_UNLOAD=1 to evade this issue, and I
suggested we probably will see other similar instances like this and should
make use of PATH to intercept via devshell wrappers - conveniently we already
have examples of this.

Here, we copy the existing "ar" example and tune it to the needs of git to
combine Richard's suggestion and mine.

As such we now also can store commit logs and use send-email with our user
specific settings, instead of "root", so in additon to fixing basic
commands like "git branch" it should also increase general usefulness.

RP: Tweaked the patch so the PATH change only applies to the devshell task
and is a generic git intercept rather than devshell specific.

RP: Also apply the PATH change to do_install tasks since that also runs under
fakeroot and several software projects inject "git describe" output into
their binaries (systemd, iputils, llvm, ipt-gpu-tools at least) causing
reproducibility issues from systems with different git versions.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>