Fix path to sysconfdir-based gitconfig in tig(1)

The ending '/etc' is supposed to be part of {sysconfdir}.
Fix HTML doc installation

Similar to Dan Church's fix in b7de063c07b0f4d5879459f84403d14b8a86a011 also
ensure that files are installed using the basename of each file.
Ignore *.swp files and restrict to only ignore top-level config.make
Add work-around for building manpages with Homebrew-based xmlto
Improve the installation instructions

... and add example config.make in contrib/.