Makefile: Fix broken Git version in nightly CI jobs
In our nightly CI jobs we test with Git's main
respectivey next
branch. Starting with 8bd7becb (Makefile: Use Gitaly's tagged Git
versions instead of ad-hoc patching, 2022-11-11), we unconditionally
write the GIT_VERSION
into Git's source tree to override the version
that e.g. git --version
would report. But when GIT_VERSION=master
,
then we fail to parse the Git version in Gitaly and thus fail all code
paths that depend on this.
Fix this by introducing a new variable that, if set, skips overriding the Git version. This variable is set in our nightly jobs now.