Replace uses of strdup with xstrdup.

Like xmalloc and xrealloc xstrdup dies with a useful message if
the native strdup() implementation returns NULL rather than a
valid pointer.

I just tried to use xstrdup in new code and found it to be missing.
However I expected it to be present as xmalloc and xrealloc are
already commonly used throughout the code.

[jc: removed the part that deals with last_XXX, which I am
 finding more and more dubious these days.]

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git.c

@@ -97,7 +97,7 @@ static char *alias_string;
 static int git_alias_config(const char *var, const char *value)
 {
 	if (!strncmp(var, "alias.", 6) && !strcmp(var + 6, alias_command)) {
-		alias_string = strdup(value);
+		alias_string = xstrdup(value);
 	}
 	return 0;
 }