strbuf: add a helper function to call the editor "on an strbuf"

This helper supports the scenario where Git has a populated `strbuf` and
wants to let the user edit it interactively.

In `git add -p`, we will use this to allow interactive hunk editing: the
diff hunks are already in memory, but we need to write them out to a
file so that an editor can be launched, then read everything back once
the user is done editing.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.h

@@ -621,6 +621,17 @@ int launch_editor(const char *path, struct strbuf *buffer,
 int launch_sequence_editor(const char *path, struct strbuf *buffer,
 			   const char *const *env);
 
+/*
+ * In contrast to `launch_editor()`, this function writes out the contents
+ * of the specified file first, then clears the `buffer`, then launches
+ * the editor and reads back in the file contents into the `buffer`.
+ * Finally, it deletes the temporary file.
+ *
+ * If `path` is relative, it refers to a file in the `.git` directory.
+ */
+int strbuf_edit_interactively(struct strbuf *buffer, const char *path,
+			      const char *const *env);
+
 void strbuf_add_lines(struct strbuf *sb,
 		      const char *prefix,
 		      const char *buf,