ui-tree.c: add support for path-selected submodule links

The current 'repo.module-link' option is sufficient when all gitlinks
in a repository can be converted to commit links in a uniform way, but
not when different submodules/paths needs different settings.

This patch adds support for 'repo.module-link.<path>', which will be
used for linking to submodules at paths matching one such entry.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgitrc.5.txt

@@ -417,6 +417,12 @@ repo.module-link::
 	formatstring are the path and SHA1 of the submodule commit. Default
 	value: <module-link>
 
+repo.module-link.<path>::
+	Text which will be used as the formatstring for a hyperlink when a
+	submodule with the specified subdirectory path is printed in a
+	directory listing. The only argument for the formatstring is the SHA1
+	of the submodule commit. Default value: none.
+
 repo.max-stats::
 	Override the default maximum statistics period. Valid values are equal
 	to the values specified for the global "max-stats" setting. Default