[PATCHv2 2/2] builtin/commit.c: drop use snprintf via dynamic allocation

[Elia Pinto <gitter.spiros@gmail.com>]

In general snprintf is bad because it may silently truncate results
if we're wrong. In this patch, instead of using xnprintf, which asserts
that we don't truncate, we are switching to dynamic allocation, so we can
avoid dealing with magic numbers in the code.

Helped-by: Jeff King <peff@peff.net> 
Signed-off-by: Elia Pinto <gitter.spiros@gmail.com>
---
This is the second version of the patch.

I have split the original commit in two, as discussed here
http://public-inbox.org/git/20161213132717.42965-1-gitter.spiros@gmail.com/.

 builtin/commit.c | 10 ++++------
 1 file changed, 4 insertions(+), 6 deletions(-)

diff --git a/builtin/commit.c b/builtin/commit.c
index 09bcc0f13..37228330c 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -1526,12 +1526,10 @@ static int git_commit_config(const char *k, const char *v, void *cb)
 static int run_rewrite_hook(const unsigned char *oldsha1,
 			    const unsigned char *newsha1)
 {
-	/* oldsha1 SP newsha1 LF NUL */
-	static char buf[2*40 + 3];
+	char *buf;
 	struct child_process proc = CHILD_PROCESS_INIT;
 	const char *argv[3];
 	int code;
-	size_t n;
 
 	argv[0] = find_hook("post-rewrite");
 	if (!argv[0])
@@ -1547,11 +1545,11 @@ static int run_rewrite_hook(const unsigned char *oldsha1,
 	code = start_command(&proc);
 	if (code)
 		return code;
-	n = snprintf(buf, sizeof(buf), "%s %s\n",
-		     sha1_to_hex(oldsha1), sha1_to_hex(newsha1));
+	buf = xstrfmt("%s %s\n", sha1_to_hex(oldsha1), sha1_to_hex(newsha1));
 	sigchain_push(SIGPIPE, SIG_IGN);
-	write_in_full(proc.in, buf, n);
+	write_in_full(proc.in, buf, strlen(buf));
 	close(proc.in);
+	free(buf);
 	sigchain_pop(SIGPIPE);
 	return finish_command(&proc);
 }
-- 
2.11.0.154.g5f5f154