Git refnames are relatively free-form and can contain all sort for
special characters, not just / and #, see
http://git-scm.com/docs/git-check-ref-format

In particular, Debian's DEP-14 standard for storing packaging in git
repositories mandates the use of the % character in tags in some
cases like debian/2%2.6-21.

Unfortunately python-gitlab currently only escapes / to %2F and in
some cases # to %23. This means that when using the commit API to
retrieve information about the debian/2%2.6-21 tag only the slash is
escaped before being inserted in the URL path and the % is left
untouched, resulting in something like
/api/v4/projects/123/repository/commits/debian%2F2%2.6-21. When
urllib3 sees that, it detects the invalid % escape and then urlencodes
the whole string, resulting in
/api/v4/projects/123/repository/commits/debian%252F2%252.6-21, where
the original / got escaped twice and produced %252F.

To avoid the issue, fully urlencode identifiers and parameters to avoid
the urllib3 auto-escaping in all cases.

I've run the unit tests but not the integration ones, so in theory this does
not break anything but I can't really confirm it. All I can firmly say is that it
fixes the issue I hit. :D