gitk: Recognize -L option

gitk: Recognize -L option

This gives line-log support to gitk, by exploiting the new support for
processing and showing "inline" diffs straight from the git-log
output.

Note that we 'set allknown 0', which is a bit counterintuitive since
this is a "known" option.  But that flag prevents gitk from thinking
it can optimize the view by running rev-list to see the topology; in
the -L case that doesn't work.

Signed-off-by: Thomas Rast <redacted>
Signed-off-by: Paul Mackerras <redacted>