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.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);
+}
 
 /*****************************************************************
  *