git/commit: remove trailing 'subst -g'

git/commit: remove trailing 'subst -g'

the subst utility no longer supports a '-g'
flag, but this was left behind in commit;

this means that the lines listing modified
files were not correctly commented in the
commit header.

This is mostly harmless, but when using an
editor like sam to edit the commit message,
the modified lines would have to be removed
manually.