A few cleanups (k88hudson#163)

* Consolidate the name of the subsystem

The name of the subsytem is considered to be 'Git' and not 'git'.
There was inconsistency in the document by referring to the subsystem
using both 'git' and 'Git'.

Consolidate the usages to 'Git' which is generally considered to be
the name of the subsystem.

* Consolidate the name of GitHub

They website is generally called 'GitHub' and not 'github'.

* Quote the git commands correctly

* Avoid fullstops in section names

This is done for the sake of consistency. Most of the section names
don't have a fullstop at the end.

So, ...

* Update the ToC

The Table of Contents seems to have been out of date with the section
titles.

So, update the ToC with 'doctoc'.

* Clarify that the changes are removed only for the previous commit

* Showcase the flexibility of `git fetch -p`

The example for that exhibits the way to 'prune' remote branches that
were deleted upstream wasn't flexible as it relied on the command
defaulting to the upstream of the current branch. This might lead
the reader into overlooking the flexibility of the `git fetch`.

Show that the 'upstream' can be mentioned in the command thus show
casing the flexibility of `git fetch`.

* Exemplify the safer version of branch deletion

It's not good for newbies to start using 'force deletion' when they
want to delete a branch as it might lead them to them into
'accidentally' deleting their branches often without merging them
into other branches or pushing them to an upstream.

So, exemplify the safer version of branch deletion (branch -d) and
warn them about what `git branch -D` does.

* Improve readability of a few phrases

It's not required to use 'git' a lot as this a document about Git,
after all.

* Use HTTPS links for sites that serve using HTTPS

* Clarify that rebasing just re-writes history

Rebasing fast-forwards when the tip of the branch is a descendent of
the tip of the upstream. In other cases it re-writes the history. This
re-write is what actually leads the user to 'force' update the remote.

So, clarify that a user has to force update only when the history is
re-written regardless of whether the branch was fast-forwarded.

* Attribute both the authors of the second edition of Pro Git

* Try a different form of emphasizing text

Capitalizing words seems to be over emphasizing words. Italicize
the words, instead to see if works.

* Mention what 'upstream' means for the sake of clarity

* Simplify the way to create a remote branch from the local one

The commands were needlessly complex by not considering the fact
that the command defaults to HEAD when no branch is specified and
changing configuration when it wasn't required.

Simplify the commands to make readers more happy!

* Remove a character cruft left over while editing

This is an instance of a carelessly edited document getting into
version control. ;)

* Improve a sentence

... by,

- expanding acronyms
- quoting a command line parameter