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>

builtin-grep.c

@@ -1048,7 +1048,7 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 				/* ignore empty line like grep does */
 				if (!buf[0])
 					continue;
-				add_pattern(&opt, strdup(buf), argv[1], ++lno,
+				add_pattern(&opt, xstrdup(buf), argv[1], ++lno,
 					    GREP_PATTERN);
 			}
 			fclose(patterns);