hg-fast-export.sh/hg-reset.sh: replace egrep with grep

According to the POSIX standard, egrep is an obsolescent equivalent
of grep -E.  In fact, the patterns actually being used with egrep do
not require use of extended regular expressions at all, so a plain
'grep' can be used rather than 'grep -E'.

Replace egrep with grep to improve compatibility across systems.