clone: add a --no-tags option to clone without tags

Add a --no-tags option to clone without fetching any tags.

Without this change there's no easy way to clone a repository without
also fetching its tags.

When supplying --single-branch the primary remote branch will be
cloned, but in addition tags will be followed & retrieved. Now
--no-tags can be added --single-branch to clone a repository without
tags, and which only tracks a single upstream branch.

This option works without --single-branch as well, and will do a
normal clone but not fetch any tags.

Many git commands pay some fixed overhead as a function of the number
of references. E.g. creating ~40k tags in linux.git will cause a
command like `git log -1 >/dev/null` to run in over a second instead
of in a matter of milliseconds, in addition numerous other things will
slow down, e.g. "git log <TAB>" with the bash completion will slowly
show ~40k references instead of 1.

The user might want to avoid all of that overhead to simply use a
repository like that to browse the "master" branch, or something like
a CI tool might want to keep that one branch up-to-date without caring
about any other references.

Without this change the only way of accomplishing this was either by
manually tweaking the config in a fresh repository:

    git init git &&
    cat >git/.git/config <<EOF &&
    [remote "origin"]
        url = git@github.com:git/git.git
        tagOpt = --no-tags
        fetch = +refs/heads/master:refs/remotes/origin/master
    [branch "master"]
        remote = origin
        merge = refs/heads/master
    EOF
    cd git &&
    git pull

Which requires hardcoding the "master" name, which may not be the main
--single-branch would have retrieved, or alternatively by setting
tagOpt=--no-tags right after cloning & deleting any existing tags:

    git clone --single-branch git@github.com:git/git.git &&
    cd git &&
    git config remote.origin.tagOpt --no-tags &&
    git tag -l | xargs git tag -d

Which of course was also subtly buggy if --branch was pointed at a
tag, leaving the user in a detached head:

    git clone --single-branch --branch v2.12.0 git@github.com:git/git.git &&
    cd git &&
    git config remote.origin.tagOpt --no-tags &&
    git tag -l | xargs git tag -d

Now all this complexity becomes the much simpler:

    git clone --single-branch --no-tags git@github.com:git/git.git

Or in the case of cloning a single tag "branch":

    git clone --single-branch --branch v2.12.0 --no-tags git@github.com:git/git.git

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1309,6 +1309,7 @@ _git_clone ()
 			--template=
 			--depth
 			--single-branch
+			--no-tags
 			--branch
 			--recurse-submodules
 			--no-single-branch