Add `execute` to run a command without a shell (and use it to u-opt git usage)

[compnerd]

This adds a new utility function execute which executes a command without the assumption that there is a POSIX-like shell being used. This functionality is needed to eventually solve #213. However, it is still useful to perform some micro-optimizations of avoiding the shell when using git.

[ocean90]

We should probably switch to @actions/exec "to execute tools in a cross platform way".

[compnerd]

@ocean90 execute is actually using spawnSync rather than exec, which doesn't have the same exact semantics.