tempfile: add mks_tempfile_dt()

Add a function to create a temporary file with a certain name in a
temporary directory created using mkdtemp(3).  Its result is more
sightly than the paths created by mks_tempfile_ts(), which include
a random prefix.  That's useful for files passed to a program that
displays their name, e.g. an external diff tool.

Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

tempfile.h

@@ -82,6 +82,7 @@ struct tempfile {
 	FILE *volatile fp;
 	volatile pid_t owner;
 	struct strbuf filename;
+	size_t directorylen;
 };
 
 /*
@@ -198,6 +199,18 @@ static inline struct tempfile *xmks_tempfile(const char *filename_template)
 	return xmks_tempfile_m(filename_template, 0600);
 }
 
+/*
+ * Attempt to create a temporary directory in $TMPDIR and to create and
+ * open a file in that new directory. Derive the directory name from the
+ * template in the manner of mkdtemp(). Arrange for directory and file
+ * to be deleted if the program exits before they are deleted
+ * explicitly. On success return a tempfile whose "filename" member
+ * contains the full path of the file and its "fd" member is open for
+ * writing the file. On error return NULL and set errno appropriately.
+ */
+struct tempfile *mks_tempfile_dt(const char *directory_template,
+				 const char *filename);
+
 /*
  * Associate a stdio stream with the temporary file (which must still
  * be open). Return `NULL` (*without* deleting the file) on error. The