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>

builtin/apply.c

@@ -1281,9 +1281,7 @@ static int parse_git_header(const char *line, int len, unsigned int size, struct
 	 */
 	patch->def_name = git_header_name(line, len);
 	if (patch->def_name && root) {
-		char *s = xmalloc(root_len + strlen(patch->def_name) + 1);
+		char *s = xstrfmt("%s%s", root, patch->def_name);
-		strcpy(s, root);
-		strcpy(s + root_len, patch->def_name);
 		free(patch->def_name);
 		patch->def_name = s;
 	}

builtin/fetch.c

@@ -1053,16 +1053,11 @@ static int fetch_one(struct remote *remote, int argc, const char **argv)
 		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);
+				refs[j++] = xstrfmt("refs/tags/%s:refs/tags/%s",
-				strcpy(ref, "refs/tags/");
+						    argv[i], argv[i]);
-				strcat(ref, argv[i]);
-				strcat(ref, ":refs/tags/");
-				strcat(ref, argv[i]);
-				refs[j++] = ref;
 			} else
 				refs[j++] = argv[i];
 		}

builtin/name-rev.c

@@ -33,10 +33,7 @@ static void name_rev(struct commit *commit,
 		return;
 
 	if (deref) {
-		char *new_name = xmalloc(strlen(tip_name)+3);
+		tip_name = xstrfmt("%s^0", tip_name);
-		strcpy(new_name, tip_name);
-		strcat(new_name, "^0");
-		tip_name = new_name;
 
 		if (generation)
 			die("generation: %d, but deref?", generation);

sha1_name.c

@@ -1252,10 +1252,7 @@ static void diagnose_invalid_sha1_path(const char *prefix,
 		die("Path '%s' exists on disk, but not in '%.*s'.",
 		    filename, object_name_len, object_name);
 	if (errno == ENOENT || errno == ENOTDIR) {
-		char *fullname = xmalloc(strlen(filename)
+		char *fullname = xstrfmt("%s%s", prefix, filename);
-					     + strlen(prefix) + 1);
-		strcpy(fullname, prefix);
-		strcat(fullname, filename);
 
 		if (!get_tree_entry(tree_sha1, fullname,
 				    sha1, &mode)) {

shell.c

@@ -46,11 +46,7 @@ static int is_valid_cmd_name(const char *cmd)
 
 static char *make_cmd(const char *prog)
 {
-	char *prefix = xmalloc((strlen(prog) + strlen(COMMAND_DIR) + 2));
+	return xstrfmt("%s/%s", COMMAND_DIR, prog);
-	strcpy(prefix, COMMAND_DIR);
-	strcat(prefix, "/");
-	strcat(prefix, prog);
-	return prefix;
 }
 
 static void cd_to_homedir(void)