The name of the hash function is "SHA-1", not "SHA1"

Use "SHA-1" instead of "SHA1" whenever we talk about the hash function.
When used as a programming symbol, we keep "SHA1".

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
glossary: improve definitions of refspec and pathspec

The exact definition of "refspec" can be found in git-fetch
and git-push manpages. So don't duplicate this here in the glossary.

Actually the definition of "pathspec" should be moved to a separate
file akin to the way it's done with "refspec". But this will only
be wortwhile when there's more to say about it. So for the time being
just improve the first sentence a little bit; fix the idendation of
the first paragraph after the bullet list and remove the one-item
list of magic signatures with its - for the user - unnecessary
introduction of "magic word 'top'".

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>