git: run in a directory given with -C option

git: run in a directory given with -C option

This is similar in spirit to "make -C dir ..." and "tar -C dir ...".

It takes more keypresses to invoke git command in a different
directory without leaving the current directory:

    1. (cd ~/foo && git status)
       git --git-dir=~/foo/.git --work-dir=~/foo status
       GIT_DIR=~/foo/.git GIT_WORK_TREE=~/foo git status
    2. (cd ../..; git grep foo)
    3. for d in d1 d2 d3; do (cd $d && git svn rebase); done

The methods shown above are acceptable for scripting but are too
cumbersome for quick command line invocations.

With this new option, the above can be done with fewer keystrokes:

    1. git -C ~/foo status
    2. git -C ../.. grep foo
    3. for d in d1 d2 d3; do git -C $d svn rebase; done

A new test script is added to verify the behavior of this option with
other path-related options like --git-dir and --work-tree.

Signed-off-by: Nazri Ramliy <ayiehere@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>