gitk: Improve display of list of nearby tags and heads

gitk: Improve display of list of nearby tags and heads

This provides a control in the preferences pane for the limit on
how many tags or heads are displayed with the commit details under
the Branch(es), Precedes and Follows headings.  This limit is now
saved in ~/.gitk so that changes are persistent.

This also applies word-wrapping to the list of tags or heads under
the Branch, Precedes and Follows headings, so that long lists are
more readable.

Signed-off-by: Paul Mackerras <redacted>