Refactor git_set_argv0_path() to git_extract_argv0_path()

Refactor git_set_argv0_path() to git_extract_argv0_path()

This commit moves the code that computes the dirname of argv[0]
from git.c's main() to git_set_argv0_path() and renames the function
to git_extract_argv0_path().  This makes the code in git.c's main
less cluttered, and we can use the dirname computation from other
main() functions too.

[ spr:
 - split Steve's original commit and wrote new commit message.
 - Integrated Johannes Schindelin's
   cca1704897e7fdb182f68d4c48a437c5d7bc5203 while rebasing onto master.
]

Signed-off-by: Steve Haslam <shaslam@lastminute.com>
Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>