git-blame --porcelain: quote filename in c-style when needed.

git-blame --porcelain: quote filename in c-style when needed.

Otherwise a pathname that has funny characters such as LF would
screw up the parsing programs of the output.

Strictly speaking, this is not backward compatible, but the
current output for pathnames that have embedded LF and such
cannot be sanely parsed anyway, and pathnames that only use
characters from the portable pathname character set won't be
affected.

Signed-off-by: Junio C Hamano <junkio@cox.net>