completion: don't modify the $cur variable in completion functions

completion: don't modify the $cur variable in completion functions

Since v1.7.4-rc0~11^2~2 (bash: get --pretty=m<tab> completion to work
with bash v4, 2010-12-02) we use _get_comp_words_by_ref() to access
completion-related variables, and the $cur variable holds the word
containing the current cursor position in all completion functions.
This $cur variable is left unchanged in most completion functions;
there are only four functions modifying its value, namely __gitcomp(),
__git_complete_revlist_file(), __git_complete_remote_or_refspec(), and
_git_config().

If this variable were never modified, then it would allow us a nice
optimisation and cleanup.  Therefore, this patch assigns $cur to an
other local variable and uses that for later modifications in those
four functions.

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