config: allow empty string as value

`git_config_set_string(config, "config.section", "")` fails when
escaping the value.

The buffer in `escape_value` is allocated without NULL-termination. And
in case of empty string 0 is passed for buffer size in `git_buf_grow`.

`git_buf_detach` returns NULL when the allocated size is 0 and that
leads to an error return in `GITERR_CHECK_ALLOC` called after
`escape_value`

The change in `config_file.c` was suggested by Russell Belfer <rb@github.com>
Merge pull request #1771 from nvloff/write_empty_config_value

config: allow setting  empty string as value