Unescape branch names in compare commit

The upgrade in GitLab v7.9 to Rails v4.1.9 caused all branch names in the compare commit mode to be escaped (e.g. `/` to `%2F`). The compare mode would not always work when comparing against branch names with a forward slash.

Opted to unescape the branch name rather than use a wildcard segment to prevent escaping slashes because it seems like a more sensible URL. The slashes in this case aren't really represented by a tree structure (e.g. /compare/one/two/branch..another/branch/here).

* Closes #1399
* Closes https://github.com/gitlabhq/gitlabhq/issues/9105

See merge request !582