This seems entirely a gitweb configuration problem, so look at /etc/gitweb.conf

Or, if you are able to navigate to a gitweb url that does show your wiki's source, fix up ikiwiki's historyurl to use the url that works.

I don't know how to navigate to a gitweb url that does show my wiki's source.

My gitweb.conf looks like this:

cat /etc/gitweb.conf
# path to git projects (<project>.git)
#$projectroot = "/var/cache/git";
$projectroot = "/home/myuser/myiki";

# directory to use for temp files
$git_temp = "/tmp";

# Change This
$site_name = "myiki";

# target of the home link on top of all pages
#$home_link = $my_uri || "/";

# html text to include at home page
#$home_text = "indextext.html";

# file with project list; by default, simply scan the projectroot dir.
#$projects_list = $projectroot;

# stylesheet to use
#@stylesheets = ("static/gitweb.css");

# javascript code for gitweb
#$javascript = "static/gitweb.js";

# logo to use
#$logo = "static/git-logo.png";

# the 'favicon'  
#$favicon = "static/git-favicon.png";

# git-diff-tree(1) options to use for generated patches
#@diff_opts = ("-M");
@diff_opts = ();