magit-change-what-branch-tracks: lose unnecessary test

Do not test for the empty string being returned by `magit-read-rev'.
If the user makes no selection that function does no longer return
the empty string but raises an error.
magit-change-what-branch-tracks: lose comments

Loose the comments inside `magit-change-what-branch-tracks'.
They would have served better as the commit message.
magit-change-what-branch-tracks: simplify by using fewer variables
magit-wash-branch-line: rename internal variable

In `magit-wash-branch-line' rename internal variable `current-string'
to `marker'.
magit-wash-branch-line: add internal variable branch-face

In `magit-wash-branch-line' add a new internal variable `branch-face'.
This allows checking only once whether the current line is about the
checked out branch.
magit-wash-branch-line: mark checked out branch with "*"

In the branch manager mark the current branch with "*", just like Git
does.  Previously "#" was used.
magit-wash-branch-line: use insert instead of insert-before-markers

When this function is called there exist no markers that need to be
moved forward.
magit-wash-branch-line: use name variable when appropriate
magit-wash-branch-line: cleanup by using insert more than once

Previously there was one call to `insert`.  Unlike `concat' that does
not treat `nil' like an empty string leading to a lot of unnecessary
THEN clauses that consisted of the explicit empty string, which blew
up the code quiet a bit.  It also made it quiet unreadable making
comments necessary; now we don't need them anymore.
magit-wash-branch-line: shorten names of internal variables
magit-wash-branch-line: loose an unnecessary comment