gitk: Make web links clickable

gitk: Make web links clickable

This makes gitk look for http or https URLs in the commit description
and make the URLs clickable.  Clicking on them will invoke an external
web browser with the URL.

The web browser command is by default "xdg-open" on Linux, "open" on
MacOS, and "cmd /c start" on Windows.  The command can be changed in
the preferences window, and it can include parameters as well as the
command name.  If it is set to the empty string then URLs will no
longer be made clickable.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>