emsdk

Git clone desired target branch directly (#1030)

When doing a git clone of a branch, instead of doing a general git clone that first would check out the default (main/master) branch and then later doing a checkout to switch to the target branch, instead specify the `--branch` option to the clone command line to immediately clone and checkout the desired final branch.

This helps first checkout runtime performance on CIs, especially when using shallow clones (GIT_CLONE_SHALLOW option).

Also this sidesteps a really odd git clone issue we are currently seeing in our CI, where it is unable to `checkout` the googletest submodule in binaryen, but would fail saying "path not found".