ui-log: try to disambiguate ref names

ui-log: try to disambiguate ref names

The 'h' querystring parameter in cgit is normally used to specify
a branch (i.e. a ref below refs/heads/), but if a repository contains
a tag with the same name as a branch the output from ui-log would use
the tag as start-revision.

This patch tries to fix the issue by checking if the specified ref is
valid as a branch name; if so, the full refname is used in the call
to setup_revisions().

Noticed-by: Takamori Yamaguchi <akschar@gmail.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>