completion: fix bugs with slashes in remote names

Previously, some calls to for-each-ref passed fixed numbers of path components to strip from refs, assuming that remote names had no slashes in them. This made completions like:

git push github/dseomn :com

Result in:

git push github/dseomn :dseomn/completion-remote-slash

With this patch, it instead results in:

git push github/dseomn :completion-remote-slash

In addition, the change to __git_dwim_remote_heads() restricts it to only list remote branches, not HEAD. I think that actually improves the completion where it's used though, since HEAD doesn't seem to be valid anyway:

$ git switch HEAD fatal: a branch is expected, got 'HEAD'

David Mandelberg (3):
completion: add helper to escape strings for fnmatch
completion: add helper to count path components
completion: fix bugs with slashes in remote names

 contrib/completion/git-completion.bash |  40 ++++-
 t/t9902-completion.sh                  | 213 ++++++++++++++++++++++---
 2 files changed, 225 insertions(+), 28 deletions(-)

base-commit: cb0ae672aeabefca9704477ea8018ac94f523970
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1901%2Fdseomn%2Fcompletion-remote-slash-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1901/dseomn/completion-remote-slash-v1
Pull-Request: https://github.com/git/git/pull/1901

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