git.c: make autocorrected aliases work

git.c: make autocorrected aliases work

help_unknown_cmd() is able to autocorrect a command to an alias, and not
only to internal or external commands. However, main() was not passing the
autocorrected command through handle_alias(), hence it failed if it was an
alias.

This commit makes the autocorrected command go through handle_alias(), once
handle_internal_command() and execv_dashed_external() have been tried. Since
this is done twice in main() now, moved that logic to a new run_argv()
function.

Also, print the same "Expansion of alias 'x' failed" message when the alias
was autocorrected, rather than a generic "Failed to run command 'x'".

Signed-off-by: Adeodato Simó <dato@net.com.org.es>
Signed-off-by: Junio C Hamano <gitster@pobox.com>