git-rerere.txt: grammatical fixups and cleanups

git-rerere.txt: grammatical fixups and cleanups

Rewrite the gc section using unresolved and resolved instead of "not
recorded". Add plurals and missing articles. Make some sentences have
consistent tense. Try and be more active by removing "that" and
simplifying sentences.

The terms "hand-resolve" and "hand resolve" were used, so just use "hand
resolve" to be more consistent.

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