gitk: collect construction of blameargs into a single conditional

gitk: collect construction of blameargs into a single conditional

The command line to invoke 'git blame' for a single line is constructed
using several if-conditionals, each with the same condition
{$from_index new {}}. Merge all of them into a single conditional.
This requires to duplicate significant parts of the command, but it
helps the next change, where we will have to deal with a nested list
structure.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Taylor Blau <me@ttaylorr.com>