cmd/go: support sha1 repos when git default is sha256
When git is recent enough (beyond 2.29), always set the --object-format
flag.
This fixes repo cloning when users have set the git configuration
init.defaultObjectFormat to sha256.
Git is planning[1] to switch the default hash function to sha256 with
the 3.0 release sometime in late 2026. (that may slip, but it's still
worth being ahead of the curve)
This change moves the version-check function from cl/698835 into
codehost/git.go so we can use it to condition setting
--object-format=sha1.
Adjust the regexp parsing git version output to handle more cases.