string-list: enable sign compare warnings check

In "add_entry", we call "get_entry_index" function to get the inserted
position. However, as the return type of "get_entry_index" function is
`int`, there is a sign compare warning when comparing the `index` with
the `list-nr` of unsigned type.

"get_entry_index" would always return unsigned index. However, the
current binary search algorithm initializes "left" to be "-1", which
necessitates the use of signed `int` return type.

The reason why we need to assign "left" to be "-1" is that in the
`while` loop, we increment "left" by 1 to determine whether the loop
should end. This design choice, while functional, forces us to use
signed arithmetic throughout the function.

To resolve this sign comparison issue, let's modify the binary search
algorithm with the following approach:

1. Initialize "left" to 0 instead of -1
2. Use `left < right` as the loop termination condition instead of
   `left + 1 < right`
3. When searching the right part, set `left = middle + 1` instead of
   `middle`

Then, we could delete "#define DISABLE_SIGN_COMPARE_WARNING" to enable
sign warnings check for "string-list".

Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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