Upgrade to new git 2.14

Most notable from the release notes

Git can now be built with PCRE v2 instead of v1 of the PCRE
   library. Replace USE_LIBPCRE=YesPlease with USE_LIBPCRE2=YesPlease
   in existing build scripts to build against the new version.  As the
   upstream PCRE maintainer has abandoned v1 maintenance for all but
   the most critical bug fixes, use of v2 is recommended.

It should be simple to upgrade considering the release notes, we should aim to do it soon.

Depends on: gitaly!483 (merged)

/cc @gitlab-build-team @joshlambert