Propagate context and ensure git commands run in request context (#17868)

This PR continues the work in #17125 by progressively ensuring that git
commands run within the request context.

This now means that the if there is a git repo already open in the context it will be used instead of reopening it.

Signed-off-by: Andrew Thornton <art27@cantab.net>

modules/git/commit_info_test.go

@@ -24,7 +24,7 @@ func cloneRepo(url, dir, name string) (string, error) {
 	if _, err := os.Stat(repoDir); err == nil {
 		return repoDir, nil
 	}
-	return repoDir, Clone(url, repoDir, CloneRepoOptions{
+	return repoDir, Clone(DefaultContext, url, repoDir, CloneRepoOptions{
 		Mirror:  false,
 		Bare:    false,
 		Quiet:   true,