docs: fix cross-directory linkgit references

docs: fix cross-directory linkgit references

Most of our documentation is in a single directory, so using
linkgit:git-config[1] just generates a relative link in the
same directory. However, this is not the case with the API
documentation in technical/*, which need to refer to
git-config from the parent directory.

We can fix this by passing a special prefix attribute when building
in a subdirectory, and respecting that prefix in our linkgit
definitions.

We only have to modify the html linkgit definition.  For
manpages, we can ignore this for two reasons:

  1. we do not generate actual links to the file in
     manpages, but instead just give the name and section of
     the linked manpage

  2. we do not currently build manpages for subdirectories,
     only html

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>