Fix various warnings

Missing header, usage of the wrong variable, and bad function signature.

https://bugzilla.gnome.org/show_bug.cgi?id=638930

tests/shell.c

@@ -8,7 +8,7 @@ typedef struct
 	GitgRepository *repository;
 } RepositoryInfo;
 
-static gboolean
+static void
 remove_all (gchar const *path,
             GError      **error)
 {

tools/gitg-shell.c

@@ -1,4 +1,5 @@
 #include <glib.h>
+#include <string.h>
 #include <stdlib.h>
 #include <libgitg/gitg-shell.h>
 #include <gio/gunixinputstream.h>
@@ -96,7 +97,7 @@ main (int argc, char *argv[])
 
 	gitg_debug_init ();
 
-	if (i == 1)
+	if (argc == 1)
 	{
 		g_print ("Please specify a command...\n");
 		return 1;