git wrapper: more careful argument stuffing
git wrapper: more careful argument stuffing

 - Use stderr for error output
 - Build git_command more careful
 - ENOENT is good enough for check of failed exec to show usage, no
   access() check needed

[jc: Originally from Alex Riesen with inputs from Sven
 Verdoolaege mixed in.]

Signed-off-by: Junio C Hamano <dc50d1021234060e53ec42a77d526afa2fe07479@cox.net>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git