Allow clone depth to be specified per project

If the clone-depth attribute is set on a project, its value will
be used to set the depth when fetching the git.  The value, if
given, must be a positive integer.

The value in the clone-depth attribute overrides any value given to
repo init via the --depth command line option.

Change-Id: I273015b3724213600b63e40cca4cafaa9f782ddf
3 files changed