build system: trim crosstool-ng commit ID when checking version

build system: trim crosstool-ng commit ID when checking version

Since 2.11.0, git does not default to 7 characters for short commit IDs,
instead automatically estimating the number of characters based on
repository size [1]. If the toolchain was compiled on a computer with
new version of git installed, commit ID will contain 8 characters, and
the check for the toolchain version will fail. As a workaround, trim the
commit ID to 7 characters when checking the version.

[1] https://github.com/git/git/commit/e6c587c733b4634030b353f4024794b08bc86892