Abort properly if source files are not under version control (FS#23122).

Remove superfluous subshell code from version control checks in
commitpkg, making it exit properly if source files are not under version
control. Also, improve correctness of sed(1) commands and use fgrep(1)
instead of grep(1) where appropriate.

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>