ci: optimize git fetch performance

This patch optimizes git fetch performance in gitlab CI with blobless clone as introduced in https://github.blog/open-source/git/get-up-to-speed-with-partial-clone-and-shallow-clone. Blobless clone is enabled for the main repo, submodules as well as the perf notes repo, reducing network traffic without loss of access to historical commits. Also enables concurrent fetching of submodules, and includes minor corrections to perf note fetching command in the testsuite driver.