Allow multiple fetch commands (or fetch and pull) to run concurrently

Git has a bug [1] whereby running multiple fetch commands at the same time
causes all of them to append their information to the .git/FETCH_HEAD file,
causing the next git pull that wants to use the information to become confused,
and show an error like "Cannot rebase onto multiple branches". This error would
occur when pressing "f" and "p" in quick succession in the files panel, but also
when pressing "p" while a background fetch happens to be running. One likely
situation for this is pressing "p" right after startup.

Since lazygit never uses the information written to .git/FETCH_HEAD, it's best
to avoid writing to it, which fixes the scenarios described above.

However, it doesn't fix the problem of repeatedly pressing "f" quickly on the
checked-out branch; since we call "git pull" in that case, the above fix doesn't
help there. We'll address this separately in another PR.

[1] See https://public-inbox.org/git/xmqqy1daffk8.fsf@gitster.g/ for more
information.