Sphinx subprocess source-path issue

[nicph]

Currently, sphinx-multiversion passes as source-path argument an absolute path to source in the temporary clone dir.

That prevents extensions like sphinxcontrib-kroki to work properly. Here is an example of the issue we can get (shortenized for readability):

/var/.../source/index.rst:: WARNING: image file not readable: ../../../Users/nicph/sphinx-multiversion-fix/build/html/master/_images/kroki-e50a2bef1e2382a2e13a968ad2866a7485b1257b.svg

This behaviour has been introduced in 537cb23, when sphinx was not run in a subprocess.

Now that sphinx is run in its own subprocess (since 427ed23), the absolute path no longer seems necessary.