git-commands: replace strsystem with "gitCommand"

This makes the use somewhat clearer and will get less unchecked Scheme
code be executed.
git-commands: Make gitCommand return whole result

Now the whole result of a git command is returned instead of
only the first line.

If the result may have more than one line the caller is responsible for
splitting the single line result into appropriate pieces.
git-commands: add gitEmail

print the email address of the latest commit