Quote LF in urls git fetch saves in FETCH_HEAD

Quote LF in urls git fetch saves in FETCH_HEAD

The fmt-merge-msg does a strong syntax checking of its input and fails
with if it is incorrect. The LF character is the only character
important for fmt-merge-msg. As the url in FETCH_HEAD plays only
informational role, a quoted representation of the url should be good
and true enough.
The url often comes from either user-editable config or command line,
so it is reasonable to expect all kinds of characters in it, including
the characters which the format of FETCH_HEAD considers special (line
separator in this case).

Noticed and reported by Hugo Mildenberger.

Signed-off-by: Alex Riesen <raa.lkml@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>