user-manual: Standardize backtick quoting

I tried to always use backticks for:
* Paths and filenames (e.g. `.git/config`)
* Compound refs (e.g. `origin/HEAD`)
* Git commands (e.g. `git log`)
* Command arguments (e.g. `--pretty`)
* URLs (e.g. `git://`), as a subset of command arguments
* Special characters (e.g. `+` in diffs).
* Config options (e.g. `branch.<name>.remote`)

Branch and tag names are sometimes set off with double quotes,
sometimes set off with backticks, and sometimes left bare.  I tried to
judge when the intention was introducing new terms or conventions
(double quotes), to reference a recently used command argument
(backticks), or to reference the abstract branch/commit (left bare).
Obviously these are not particularly crisp definitions, so my
decisions are fairly arbitrary ;).  When a reference had already been
introduced, I changed further double-quoted instances to backticked
instances.

When new backticks increased the length of a line beyond others in
that block, I re-wrapped blocks to 72 columns.

Signed-off-by: W. Trevor King <wking@tremily.us>
Signed-off-by: Junio C Hamano <gitster@pobox.com>