_git: fix __git_committish_range_{first,last} and __git_is_committish_range

- Ranges with 3 dots would always fail, because the non-greedy
  expansion  %..(.|)* in __git_committish_range_first would only
  remove '..' and never three dots. 'a...b' would end up in 'a.'.
  Use ${${1%..*}%.} instead.
- Use a similar approach for __git_committish_range_last.
- Wrap them in another expansion to replace empty results with 'HEAD'.
  Git man-page states omitted range ending are being replaced with
  HEAD. This rule has to be followed to make completions like
  'git log foo.. -- <tab>' work properly.
- Add an additional check to make sure none of the extracted first/last
  parts contain additional '..' in invalied ranges such as 'a..b..c'.
  This gets rid of the 'TODO:' and ideally saves a few unneded
  calls to git rev-parse.
_git: diff: refactor and fix wrong completions

Before this, there were several cases where the completion
would offer the wrong things:

$ git diff branch -- <tab>

would try to complete "changed in workdir files", but needs to
complete all "tree files in HEAD".

$ git diff --cached -- file1 file2 <tab>
would try to complete "changed in workdir files" but needs to
complete "changed in index files".

...

After this change all possible combinations are taken into
account and completion should work properly.
_git: fix __git_submodules to only use the actual name of the submodule

The output of 'submodule status' is

Xsha1 name (describe)

X being one of -,+,U,[space]

We are only interested in the name part and not the whole line.

Fix the parameter expansions accordingly.
_git: completion updates up to latest git v1.9.0

- cherry-pick: allow commit ranges to be specified
- push: option is called --set-upstream and not --set-upstream-to
- status: offer -b,--branch when --porcelain or -z are given
  --porcelain and -z don't show the branch info by default, so it
  is needed to offer --branch for those options, too.
- checkout: add '--detach' option
- checkout: offer '-l' option when '--orphan' was given
- show-ref: update wording of --head and --heads according to man-page
- config: update default value for color.ui
  color-bool was changed to accept a default value as $parts[5]
- add support for 'cygstart' as a valid builtin browser on cygwin
- rebase: add suuport for --autostash and config.autostash
- update builtin browser list
- grep: add --{no,}textconv
- check-ignore: add --no-index
- update-ref: add --stdin -z
- add -C
- pull: add support for --rebase=preserve
- config: add support for http.*. options
- blame: -L can be given multiple times
- config: add support for http.savecookies
- push: add support for --force-with-lease
- diff: --diff-filter: allow lower-case variants (all-but ... specs)
- config: add support for 'fetch.prune' and 'remote.*.prune'
- check-ignore: -z: update message, check-attr: add -z
- config: add diff.orderfile
- revision options: add --exclude
- revision options: add --ignore-missing
- revision options: add --bisect
- rev-parse: add --stuck-long
- merge-base: add --fork-point
- config: implement submodule.*.update completion
- send-email: add --smtp-ssl-cert-path and config options