gitcl: log output of failed git commands.

[aznashwan]

Hello, and thanks a bunch for this awesome crate!

This patch makes any git commands in the vergen-gitcl subcrate log their stdout/stderr on failure to provide more background.

Backstory:

I've just wasted a couple of hours debugging vergen telling me that it was "not within a suitable 'git' worktree!" when it clearly was...

The root cause of the issue turned out to be that the rust-build.action I was using had "dubious ownership" on the .git directory, as it is a container-based GitHub action where the $GITHUB_WORKSPACE is bind-mounted within the container and thus has differing user IDs.

The effect of this was that some git subcommands (most notably rev-parse) would error out like so:

$ git rev-parse --is-inside-work-tree
fatal: detected dubious ownership in repository at '/github/workspace'
To add an exception for this directory, call:

	git config --global --add safe.directory /github/workspace

While vergen would only output :

...
  --- stderr
  Error: not within a suitable 'git' worktree!

I have proposed a fix on the rust-build action's side but had vergen just exposed the stderr of the git rev-parse command to me from the get-go, it would have saved me hours of debugging (reproducing the setup with a container-based GitHub action is tedious to say the least).

Fix notes.

I've simply went with manually-formatted eprintln!() statements in the run_cmd() method used throughout the rest of the code to log any git command failures since I don't consider adding a whole logging crate tree necessary, but I could if it's deemed it is.