When renaming (or deleting) a section of configuration, Git uses the
function `git_config_copy_or_rename_section_in_file()` to rewrite the
configuration file after applying the rename or deletion to the given
section.

To do this, Git repeatedly calls `fgets()` to read the existing
configuration data into a fixed size buffer.

When the configuration value under `old_name` exceeds the size of the
buffer, we will call `fgets()` an additional time even if there is no
newline in the configuration file, since our read length is capped at
`sizeof(buf)`.

If the first character of the buffer (after zero or more characters
satisfying `isspace()`) is a '[', Git will incorrectly treat it as
beginning a new section when the original section is being removed. In
other words, a configuration value satisfying this criteria can
incorrectly be considered as a new secftion instead of a variable in the
original section.

Avoid this issue by using a variable-width buffer in the form of a
strbuf rather than a fixed-with region on the stack. A couple of small
points worth noting:

  - Using a strbuf will cause us to allocate arbitrary sizes to match
    the length of each line.  In practice, we don't expect any
    reasonable configuration files to have lines that long, and a
    bandaid will be introduced in a later patch to ensure that this is
    the case.

  - We are using strbuf_getwholeline() here instead of strbuf_getline()
    in order to match `fgets()`'s behavior of leaving the trailing LF
    character on the buffer (as well as a trailing NUL).

    This could be changed later, but using strbuf_getwholeline() changes
    the least about this function's implementation, so it is picked as
    the safest path.

  - It is temping to want to replace the loop to skip over characters
    matching isspace() at the beginning of the buffer with a convenience
    function like `strbuf_ltrim()`. But this is the wrong approach for a
    couple of reasons:

    First, it involves a potentially large and expensive `memmove()`
    which we would like to avoid. Second, and more importantly, we also
    *do* want to preserve those spaces to avoid changing the output of
    other sections.

In all, this patch is a minimal replacement of the fixed-width buffer in
`git_config_copy_or_rename_section_in_file()` to instead use a `struct
strbuf`.

Reported-by: André Baptista <andre@ethiack.com>
Reported-by: Vítor Pinho <vitor@ethiack.com>
Helped-by: Patrick Steinhardt <ps@pks.im>
Co-authored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Taylor Blau <me@ttaylorr.com>