internal/stdlib: remove dependency on github.com/go-git/go-git/v5

Instead of using the in-memory git library implementation, just run
all git operations using git commands.

Add the MustHaveExecPath function from the internal/testenv library in
the Go repo to our internal/testenv (with a modification to drop
MustHaveExec because we don't really need it) so that we can check
that git exists on the test platform before running tests that use
this code because now we need a git binary to do git operations.

For #61399
For #61341

Change-Id: I53441ee1638353a23935ba6a5ac70e2965b546f9
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/510415
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>
kokoro-CI: kokoro <noreply+kokoro@google.com>
Run-TryBot: Michael Matloob <matloob@golang.org>

0 files changed, 0 insertions, 0 deletions