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-compat-util.h

@@ -84,6 +84,14 @@ extern char *gitstrcasestr(const char *haystack, const char *needle);
 extern size_t gitstrlcpy(char *, const char *, size_t);
 #endif
 
+static inline char* xstrdup(const char *str)
+{
+	char *ret = strdup(str);
+	if (!ret)
+		die("Out of memory, strdup failed");
+	return ret;
+}
+
 static inline void *xmalloc(size_t size)
 {
 	void *ret = malloc(size);