completion: fix completion after 'git --option <TAB>'

completion: fix completion after 'git --option <TAB>'

The bash completion doesn't work when certain options to git itself are
specified, e.g. 'git --no-pager <TAB>' errors out with

    error: invalid key: alias.--no-pager

The main _git() completion function finds out the git command name by
looping through all the words on the command line and searching for
the first word that is not a known option for the git command.

Unfortunately the list of known git options was not updated in a long
time, and newer options are not skipped but mistaken for a git command.
Such a misrecognized "command" is then passed to __git_aliased_command(),
which in turn passes it to a 'git config' query, hence the error.

Currently the following options are misrecognized for a git command:

  -c --no-pager --exec-path --html-path --man-path --info-path
  --no-replace-objects --work-tree= --namespace=

To fix this we could just update the list of options to be skipped,
but the same issue will likely arise, if the git command learns a new
option in the future.  Therefore, to make it more future proof against
new options, this patch changes that loop to skip all option-looking
words, i.e. words starting with a dash.

We also have to handle the '-c' option specially, because it takes a
configutation parameter in a separate word, which must be skipped,
too.

[fc: added tests]

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>