completion: optionally disable checkout DWIM

When we complete branch names for "git checkout", we also complete remote branch names that could trigger the DWIM behavior. Depending on your workflow and project, this can be either convenient or annoying. For instance, my clone of gitster.git contains 74 local "jk/*" branches, but origin contains another 147. When I want to checkout a local branch but can't quite remember the name, tab completion shows me 251 entries. And worse, for a topic that has been picked up for pu, the upstream branch name is likely to be similar to mine, leading to a high probability that I pick the wrong one and accidentally create a new branch. This patch adds a way for the user to tell the completion code not to include DWIM suggestions for checkout. This can already be done by typing: git checkout --no-guess jk/<TAB> but that's rather cumbersome. The downside, of course, is that you no longer get completion support when you _do_ want to invoke the DWIM behavior. But depending on your workflow, that may not be a big loss (for instance, in git.git I am much more likely to want to detach, so I'd type "git checkout origin/jk/<TAB>" anyway). Signed-off-by: Jeff King <peff@peff.net> Reviewed-by: SZEDER Gábor <szeder.dev@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>