Makefile: cleanup whitespace
magit-diff-wash-tag: new function

The command `magit-show-commit' can now show tags too, previously
it showed information about "TAG^{commit}" instead of "TAG".  This
is implemented in the new function `magit-diff-wash-tag' which is
called from `magit-diff-wash-revision'.
magit-revision-insert-related-refs: truncate number of branches
rename magit-list-contain{ed => ing}-branches