doc/SubmittingPatches: improve text formatting

doc/SubmittingPatches: improve text formatting

049e64aa50 ("Documentation: convert SubmittingPatches to AsciiDoc",
2017-11-12) changed the `git blame` and `git shortlog` examples given in
the section on sending your patches.

In order to italicize the `$path` argument the commands are enclosed in
plus characters as opposed to backticks.  The difference between the
quoting methods is that backtick enclosed text is not subject to further
expansion.  This formatting makes reading SubmittingPatches in a git
clone a little more difficult.  In addition to the underscores around
`$path` the `--` chars in `git shortlog --no-merges` must be replaced
with `{litdd}`.

Use backticks to quote these commands.  The italicized `$path` is lost
from the html version but the commands can be read (and copied) more
easily by users reading the text version.  These readers are more likely
to use the commands while submitting patches.  Make it easier for them.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>