Refactor, adding prepare_git_cmd(const char **argv)

Refactor, adding prepare_git_cmd(const char **argv)

prepare_git_cmd(const char **argv) adds a first entry "git" to
the array argv.  The new array is allocated on the heap.  It's
the caller's responsibility to release it with free().  The code
was already present in execv_git_cmd() but could not be used from
outside.  Now it can also be called for preparing the command list
in the MinGW codepath in run-command.c.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>