gitk: fix highlighted remote prefix of branches with directories

gitk: fix highlighted remote prefix of branches with directories

The decoration of a remote ref is colored in two parts: (1) the prefix
that mentions the remove (including "remote/"); and (2) the branch name.
To extract the prefix from the ref name, a regular expression is used.
However, the expression is not restrictive enough: it picks everything
before the last slash character as prefix, so that, for example, the
ref name "remotes/orgin/ml/themes" is split into "remotes/origin/ml" and
"themes".

Tighten the regular expression so that only the name of the remote is
pulled into the prefix, but no part of the branch name. This gives the
desired result in the example: "remotes/origin" and "ml/themes".

Signed-off-by: Johannes Sixt <j6t@kdbg.org>