docs: use "character encoding" to refer to commit-object encoding

docs: use "character encoding" to refer to commit-object encoding

The word "encoding" can mean a lot of things (e.g., base64 or
quoted-printable encoding in emails, HTML entities, URL encoding, and so
on). The documentation for i18n.commitEncoding and i18n.logOutputEncoding
uses the phrase "character encoding" to make this more clear.

Let's use that phrase in other places to make it clear what kind of
encoding we are talking about. This patch covers the gui.encoding
option, as well as the --encoding option for git-log, etc (in this
latter case, I word-smithed the sentence a little at the same time).
That, coupled with the mention of iconv in the --encoding description,
should make this more clear.

The other spot I looked at is the working-tree-encoding section of
gitattributes(5). But it gives specific examples of encodings that I
think make the meaning pretty clear already.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>