Always build Git with PCRE2 support

Gitaly requires support for PCRE2 in Git for the RepositoryService's search functionality, but the self-built version of Git that Gitaly started to provide currently doesn't build it with PCRE2 support.

Fix this by adding USE_PCRE2=YesPlease to Git's build options. To detect missing PCRE2 support early, this commit also adapts tests to unconditionally check for PCRE2 support being present.