use xstrfmt to replace xmalloc + strcpy/strcat

It's easy to get manual allocation calculations wrong, and
the use of strcpy/strcat raise red flags for people looking
for buffer overflows (though in this case each site was
fine).

It's also shorter to use xstrfmt, and the printf-format
tends to be easier for a reader to see what the final string
will look like.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/apply.c b/builtin/apply.c
index 9c5724e..b796910 100644
--- a/builtin/apply.c
+++ b/builtin/apply.c
@@ -1281,9 +1281,7 @@
 	 */
 	patch->def_name = git_header_name(line, len);
 	if (patch->def_name && root) {
-		char *s = xmalloc(root_len + strlen(patch->def_name) + 1);
-		strcpy(s, root);
-		strcpy(s + root_len, patch->def_name);
+		char *s = xstrfmt("%s%s", root, patch->def_name);
 		free(patch->def_name);
 		patch->def_name = s;
 	}
diff --git a/builtin/fetch.c b/builtin/fetch.c
index 55f457c..40d989f 100644
--- a/builtin/fetch.c
+++ b/builtin/fetch.c
@@ -1053,16 +1053,11 @@
 		refs = xcalloc(argc + 1, sizeof(const char *));
 		for (i = 0; i < argc; i++) {
 			if (!strcmp(argv[i], "tag")) {
-				char *ref;
 				i++;
 				if (i >= argc)
 					die(_("You need to specify a tag name."));
-				ref = xmalloc(strlen(argv[i]) * 2 + 22);
-				strcpy(ref, "refs/tags/");
-				strcat(ref, argv[i]);
-				strcat(ref, ":refs/tags/");
-				strcat(ref, argv[i]);
-				refs[j++] = ref;
+				refs[j++] = xstrfmt("refs/tags/%s:refs/tags/%s",
+						    argv[i], argv[i]);
 			} else
 				refs[j++] = argv[i];
 		}
diff --git a/builtin/name-rev.c b/builtin/name-rev.c
index c824d4e..3c8f319 100644
--- a/builtin/name-rev.c
+++ b/builtin/name-rev.c
@@ -33,10 +33,7 @@
 		return;
 
 	if (deref) {
-		char *new_name = xmalloc(strlen(tip_name)+3);
-		strcpy(new_name, tip_name);
-		strcat(new_name, "^0");
-		tip_name = new_name;
+		tip_name = xstrfmt("%s^0", tip_name);
 
 		if (generation)
 			die("generation: %d, but deref?", generation);