buffer: add git_buf_clear

Set the size to zero so the memory that has already been allocated
can be reused

Signed-off-by: Carlos Martín Nieto <carlos@cmartin.tk>

src/buffer.c

@@ -93,3 +93,8 @@ void git_buf_free(git_buf *buf)
 {
 	free(buf->ptr);
 }
+
+void git_buf_clear(git_buf *buf)
+{
+	buf->size = 0;
+}

src/buffer.h

@@ -18,6 +18,7 @@ void git_buf_puts(git_buf *buf, const char *string);
 void git_buf_printf(git_buf *buf, const char *format, ...) GIT_FORMAT_PRINTF(2, 3);
 const char *git_buf_cstr(git_buf *buf);
 void git_buf_free(git_buf *buf);
+void git_buf_clear(git_buf *buf);
 
 #define git_buf_PUTS(buf, str) git_buf_put(buf, str, sizeof(str) - 1)