gitk: use single blamestuff for all show_line_source{} calls

There seems to be no point to search for several origins at once.
Probably is is not even fully working (because there is one blameinst),
but blamestuff for some reason is an array. Also, it is not cleaned
after blame is completed.

Signed-off-by: Max Kirillov <max@max630.net>
gitk: refactor: separate generic hunk parsing out of find_hunk_blamespecs{}

For requesting a region blame, it is necessary to parse a hunk and
find the region in the parent file corresponding to the selected region.
There is already hunk parsing functionality in find_hunk_blamespec{},
but it returns only information for a single line.

The new function, resolve_hunk_lines{}, scans the hunk once and returns
for all hunk lines between $start_diffline and $end_diffline, in which parent
each of them exists and which is its number there.

Signed-off-by: Max Kirillov <max@max630.net>
gitk: refactor: separate io from logic in the searching origin of line

The pattern of maintaining blame command and collecting output
can be reused for searching of latest change to region.
It still can use the blame's global variables, because the two
search commands should not run concurrently as well as two instances
of blame.

Signed-off-by: Max Kirillov <max@max630.net>
gitk: show latest change to region

Add a new command to the diffmenu, "Show the latest change of selected
region".  The menu command picks selection, and if it exists and covers
a single hunk, locates the latest change which has been made to the
selected lines in the file.

The menu command is disabled if the region blame is impossible, for
example if nothing is selected or the selection does not lie fully in a
single diff hunk.

The search is implemented by use of "git log -L..." command. Unlike git
blame, it can locate merges which brings together changes to the region
from different branches. This is what is desired, actually.

Unfortunally, using git log -L for finding a single line origin is
suboptimal, because (a) it does not support the "--contents" commandline
argument, or any other way to blame uncommitted changes, and (b) it is
noticeably slower. Hopely in some future git log -L will be mature
enough to be used for picking the single line origin, for now the best
option is to implement region logic separately, reusing the blame's basic io.

For diffs, the first parent is always searched. This decision is quite
voluntary, just to avoid complications to UI.

Signed-off-by: Max Kirillov <max@max630.net>
Merge older version after rebase

Changes:
 * rebase to latest gitk
 * squash 'gitk: show latest change to region' with switching to patch mode