gitk: Unify handling of merge diffs with normal 2-way diffs

gitk: Unify handling of merge diffs with normal 2-way diffs

This adds code to getblobdiffline to make it able to recognize and
display merge diffs (i.e. N-way diffs for N >= 3) as well as normal
two-way diffs.  This means that it can also correctly display the
3-way diff of the local changes when the git repository is in the
middle of a merge with conflicts.

This also removes getmergediffline and changes mergediff to invoke
getblobdiffline rather than getmergediffline.

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