fetch --all/--multiple: keep all the fetched branch information

fetch --all/--multiple: keep all the fetched branch information

Since "git fetch" learned "--all" and "--multiple" options, it has become
tempting for users to say "git pull --all".  Even though it may fetch from
remotes that do not need to be fetched from for merging with the current
branch, it is handy.

"git fetch" however clears the list of fetched branches every time it
contacts a different remote.  Unless the current branch is configured to
merge with a branch from a remote that happens to be the last in the list
of remotes that are contacted, "git pull" that fetches from multiple
remotes will not be able to find the branch it should be merging with.

Make "fetch" clear FETCH_HEAD (unless --append is given) and then append
the list of branches fetched to it (even when --append is not given).  That
way, "pull" will be able to find the data for the branch being merged in
FETCH_HEAD no matter where the remote appears in the list of remotes to be
contacted by "git fetch".

Reported-by: Michael Lukashov
Signed-off-by: Junio C Hamano <gitster@pobox.com>