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>