Check for shallow clone in doxia-site-scm-context

### Affected version

2.1.0

### Bug description

Shallow clones may return incorrect modification dates (compare with https://github.com/apache/maven-scm/issues/1378). Therefore there should be a warn or even an exception if shallow clones are detected in https://github.com/apache/maven-doxia-sitetools/blob/96ce057a6d82b462c892b73ff8ccc0631ee697a7/doxia-site-scm-context/src/main/java/org/apache/maven/doxia/scm/siterenderer/ScmAttributesContextCustomizer.java#L98.

Start in doxia-site-scm-context/src/main/java/org/apache/maven/doxia/scm/siterenderer/ScmAttributesContextCustomizer.java around line 98, and compare the shallow-clone behavior with maven-scm issue 1378. Determine how shallow clones are detected and make the context customizer warn or raise an exception when one is found; done means incorrect modification dates are no longer silently accepted.

Written by the indexing model from the issue text.