`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>
Passing NULL is non-sensical. The error message leaves to be desired,
though, as it leaks internal implementation details. Catch it at the
`git_config_set_string` level and set an appropriate error message.
Added `struct git_config_entry`: a git_config_entry contains the key, the value, and the config file level from which a config element was found.
Added `git_config_open_level`: build a single-level focused config object from a multi-level one.
We are now storing `git_config_entry`s in the khash of the config_file
write_section() mistakenly treated is input as the whole variable name
instead of simply the section (and possibly subsection) and would
confuse "section.subsection" as a section plus variable name and
produce a wrong section header.
Fix this and include a test for writing "section.subsection.var" and
reading it from the file.
