`magit-run-git-gui-blame': s/magit-start-process/start-file-process/

The "minimal-emacs" branch removed a number of compatibility functions,
among which `magit-start-process', replacing it with `start-file-process'.

Between the time that branch was created (commit 83dac351) and merged
(commit b872cbf7), `magit-run-git-gui-blame' was added (commit 007da202),
which also makes use of `magit-start-process', and this wasn't corrected
when merging in the "minimal-emacs" branch.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-run-git-gui-blame': s/Git Blame/Git Gui Blame/

Rename subprocess to "Git Gui Blame", to be consistent with
`magit-run-git-gui'.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-run-git-gui-blame': pass options *before* rev and filename

When running `magit-run-git-gui-blame' and selecting the file that is
being visited in the current buffer, git-gui(1) is supposed to scroll
the view to center on the line number at which point currently resides.

For this to work, we need to pass the '--line=' option *before* the rev
and filename.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #837 from praet/git-gui-blame

Git Gui Blame fixes