completion: make compatible with zsh

completion: make compatible with zsh

Modify git-completion.bash so that it also works with zsh when using
bashcompinit.  In particular:

declare -F
    Zsh doesn't have the same 'declare -F' as bash, but 'declare -f'
    is the same, and it works just as well for our purposes.

${var:2}
    Zsh does not implement ${var:2} to skip the first 2 characters, but
    ${var#??} works in both shells to replace the first 2 characters
    with nothing.  Thanks to Jonathan Nieder for the suggestion.

for (( n=1; "$n" ... ))
    Zsh does not allow "$var" in arithmetic loops.  Instead, pre-compute
    the endpoint and use the variables without $'s or quotes.

shopt
    Zsh uses 'setopt', which has a different syntax than 'shopt'.  Since
    'shopt' is used infrequently in git-completion, we provide
    a bare-bones emulation.

emulate -L bash
KSH_TYPESET
    Zsh offers bash emulation, which turns on a set of features to
    closely resemble bash. In particular, this enables SH_WORDSPLIT,
    which splits scalar variables on word boundaries in 'for' loops.
    We also need to set KSH_TYPESET, to fix "local var=$(echo foo bar)"
    issues.

The last set of options are turned on only in _git and _gitk.  Some of
the sub-functions may not work correctly if called directly.

Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>