Simplify Makefile configuration for Git

Simplifies the Makefile somewhat by removing the OVERRIDE_GIT_VERSION variable. As a result, there are only two Makefile variables which can be tweaked to change the Git version in use. We'll support the following scenarios:

No custom GIT_VERSION. Tagged version for bundled Git.

GIT_VERSION ?=
GIT_VERSION_2_45 ?= v2.45.2

Git v2.45.2 is compiled and embedded into the Gitaly binary. The Git version reported at startup will be v2.45.2.

No custom GIT_VERSION. Commit SHA for bundled Git.

GIT_VERSION ?=
GIT_VERSION_2_45 ?= f8c58f24cc37086619967959fc75c1db34583dba

Git f8c58f24cc37086619967959fc75c1db34583dba is compiled and embedded into the Gitaly binary. The Git version reported at startup will be v2.45.1.

Custom GIT_VERSION. Bundled Git is ignored.

GIT_VERSION ?= next
GIT_VERSION_2_45 ?= v2.45.2

Git v2.45.2 and the next branch will be compiled. v2.45.2 will be embedded into the Gitaly binary while next will remain in the _build/deps/git-distribution directory. Because WITH_BUNDLED_GIT will be unset, GITALY_TESTING_GIT_BINARY will be set by the Makefile to point to the Git binary inside git-distribution. This gets picked up at runtime as the Git path to use. The Git version reported at startup will be v99.99.99.