Fix %(commit) assignment in the diff view

Fix %(commit) assignment in the diff view

Since commit 0c278556e52b9b78fde9f089cab755a6366d9ce6, the %(commit)
variable is badly overwritten with part of the commit title when we
highlight this line. For instance, browsing a commit message like this:

    linux: add support for [...]

    This text explains the patch
    [...]

will result in the %(commit) variable to be "ux:".
(the offset corresponds to strlen("commit ") which is LINE_COMMIT).

This patch reworks commit 0c278556e52b9b78fde9f089cab755a6366d9ce6:
"Fix regression causing incorect coloring of the commit title", to
set the line type to LINE_DEFAULT instead of LINE_COMMIT.

Closes #179

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Jonas Fonseca <fonseca@diku.dk>