use xstrfmt in favor of manual size calculations

In many parts of the code, we do an ugly and error-prone
malloc like:

  const char *fmt = "something %s";
  buf = xmalloc(strlen(foo) + 10 + 1);
  sprintf(buf, fmt, foo);

This makes the code brittle, and if we ever get the
allocation wrong, is a potential heap overflow. Let's
instead favor xstrfmt, which handles the allocation
automatically, and makes the code shorter and more readable.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

remote.c

@@ -170,7 +170,6 @@ static struct branch *make_branch(const char *name, int len)
 {
 	struct branch *ret;
 	int i;
-	char *refname;
 
 	for (i = 0; i < branches_nr; i++) {
 		if (len ? (!strncmp(name, branches[i]->name, len) &&
@@ -186,10 +185,7 @@ static struct branch *make_branch(const char *name, int len)
 		ret->name = xstrndup(name, len);
 	else
 		ret->name = xstrdup(name);
-	refname = xmalloc(strlen(name) + strlen("refs/heads/") + 1);
-	strcpy(refname, "refs/heads/");
-	strcpy(refname + strlen("refs/heads/"), ret->name);
-	ret->refname = refname;
+	ret->refname = xstrfmt("refs/heads/%s", ret->name);
 
 	return ret;
 }

unpack-trees.c

@@ -56,17 +56,15 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 	int i;
 	const char **msgs = opts->msgs;
 	const char *msg;
-	char *tmp;
 	const char *cmd2 = strcmp(cmd, "checkout") ? cmd : "switch branches";
+
 	if (advice_commit_before_merge)
 		msg = "Your local changes to the following files would be overwritten by %s:\n%%s"
 			"Please, commit your changes or stash them before you can %s.";
 	else
 		msg = "Your local changes to the following files would be overwritten by %s:\n%%s";
-	tmp = xmalloc(strlen(msg) + strlen(cmd) + strlen(cmd2) - 2);
-	sprintf(tmp, msg, cmd, cmd2);
-	msgs[ERROR_WOULD_OVERWRITE] = tmp;
-	msgs[ERROR_NOT_UPTODATE_FILE] = tmp;
+	msgs[ERROR_WOULD_OVERWRITE] = msgs[ERROR_NOT_UPTODATE_FILE] =
+		xstrfmt(msg, cmd, cmd2);
 
 	msgs[ERROR_NOT_UPTODATE_DIR] =
 		"Updating the following directories would lose untracked files in it:\n%s";
@@ -76,12 +74,9 @@ void setup_unpack_trees_porcelain(struct unpack_trees_options *opts,
 			"Please move or remove them before you can %s.";
 	else
 		msg = "The following untracked working tree files would be %s by %s:\n%%s";
-	tmp = xmalloc(strlen(msg) + strlen(cmd) + strlen("removed") + strlen(cmd2) - 4);
-	sprintf(tmp, msg, "removed", cmd, cmd2);
-	msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] = tmp;
-	tmp = xmalloc(strlen(msg) + strlen(cmd) + strlen("overwritten") + strlen(cmd2) - 4);
-	sprintf(tmp, msg, "overwritten", cmd, cmd2);
-	msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] = tmp;
+
+	msgs[ERROR_WOULD_LOSE_UNTRACKED_REMOVED] = xstrfmt(msg, "removed", cmd, cmd2);
+	msgs[ERROR_WOULD_LOSE_UNTRACKED_OVERWRITTEN] = xstrfmt(msg, "overwritten", cmd, cmd2);
 
 	/*
 	 * Special case: ERROR_BIND_OVERLAP refers to a pair of paths, we