teach convert_to_git a "dry run" mode

Some callers may want to know whether convert_to_git will
actually do anything before performing the conversion
itself (e.g., to decide whether to stream or handle blobs
in-core). This patch lets callers specify the dry run mode
by passing a NULL destination buffer. The return value,
instead of indicating whether conversion happened, will
indicate whether conversion would occur.

For readability, we also include a wrapper function which
makes it more obvious we are not actually performing the
conversion.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

convert.c

@@ -231,6 +231,13 @@ static int crlf_to_git(const char *path, const char *src, size_t len,
 	if (!stats.cr)
 		return 0;
 
+	/*
+	 * At this point all of our source analysis is done, and we are sure we
+	 * would convert. If we are in dry-run mode, we can give an answer.
+	 */
+	if (!buf)
+		return 1;
+
 	/* only grow if not in place */
 	if (strbuf_avail(buf) + buf->len < len)
 		strbuf_grow(buf, len - buf->len);
@@ -391,6 +398,9 @@ static int apply_filter(const char *path, const char *src, size_t len,
 	if (!cmd)
 		return 0;
 
+	if (!dst)
+		return 1;
+
 	memset(&async, 0, sizeof(async));
 	async.proc = filter_buffer;
 	async.data = &params;
@@ -525,6 +535,9 @@ static int ident_to_git(const char *path, const char *src, size_t len,
 	if (!ident || !count_ident(src, len))
 		return 0;
 
+	if (!buf)
+		return 1;
+
 	/* only grow if not in place */
 	if (strbuf_avail(buf) + buf->len < len)
 		strbuf_grow(buf, len - buf->len);
@@ -754,13 +767,13 @@ int convert_to_git(const char *path, const char *src, size_t len,
 		filter = ca.drv->clean;
 
 	ret |= apply_filter(path, src, len, dst, filter);
-	if (ret) {
+	if (ret && dst) {
 		src = dst->buf;
 		len = dst->len;
 	}
 	ca.crlf_action = input_crlf_action(ca.crlf_action, ca.eol_attr);
 	ret |= crlf_to_git(path, src, len, dst, ca.crlf_action, checksafe);
-	if (ret) {
+	if (ret && dst) {
 		src = dst->buf;
 		len = dst->len;
 	}

convert.h

@@ -40,6 +40,11 @@ extern int convert_to_working_tree(const char *path, const char *src,
 				   size_t len, struct strbuf *dst);
 extern int renormalize_buffer(const char *path, const char *src, size_t len,
 			      struct strbuf *dst);
+static inline int would_convert_to_git(const char *path, const char *src,
+				       size_t len, enum safe_crlf checksafe)
+{
+	return convert_to_git(path, src, len, NULL, checksafe);
+}
 
 /*****************************************************************
  *