sha1-file: rename algorithm to "sha1"

sha1-file: rename algorithm to "sha1"

The transition plan anticipates us using a syntax such as "^{sha1}" for
disambiguation.  Since this is a syntax some people will be typing a
lot, it makes sense to provide a short, easy-to-type syntax.  Omitting
the dash doesn't create any ambiguity; however, it does make the syntax
shorter and easier to type, especially for touch typists.  In addition,
the transition plan already uses "sha1" in this context.

Rename the name of SHA-1 implementation to "sha1".

Note that this change creates no backwards compatibility concerns, since
we haven't yet used this field in any configuration settings.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>