Makefile: fix the "built from commit" code

Makefile: fix the "built from commit" code

In ed32b788c06 (version --build-options: report commit, too, if
possible, 2017-12-15), we introduced code to let `git version
--build-options` report the current commit from which the binaries were
built, if any.

To prevent erroneous commits from being reported (e.g. when unpacking
Git's source code from a .tar.gz file into a subdirectory of a different
Git project, as e.g. git_osx_installer does), we painstakingly set
GIT_CEILING_DIRECTORIES when trying to determine the current commit.

Except that we got the quoting wrong, and that variable therefore does
not have the desired effect.

The issue is that the $(shell) is resolved before the output is stuffed
into the command-line with -DGIT_BUILT_FROM_COMMIT, and therefore is
*not* inside quotes. And thus backslashing the quotes is wrong, as the
quote gets literally inserted into the CEILING_DIRECTORIES variable.

Let's fix that quoting, and while at it, also suppress the unhelpful
message

fatal: not a git repository (or any of the parent directories): .git

that gets printed to stderr if no current commit could be determined,
and might scare the occasional developer who simply tries to build Git
from scratch.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>