Handling of OUTDIR when missing / mention in usage
gitpkg.sh: Better usage description, "--help"/"-h" support
Some documentation cleanups and formatting fixes
gp_mkpkg: Use sys.exit(1) to exit with failure

system.exit() doesn't exist, it just happened to have
the same effect (interpreter exits because of a NameError).
gp_mkpkg: Centralized error reporting

Error messages should be printed to stderr.
gp_mkpkg: New git() function for getting git output

Allow overriding of the "git" command used via the environment
variable "GIT" (default changed from "/usr/bin/git" to "git").

Central git() function that will call git with the given arguments
and return stdout as string.

Cleaner (not necessarily more efficient) implementation of
count_commits() that makes use of the new git() function.

Use git() where possible, GIT_CMD where it's not (yet) possible.