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>