Previously, we were using the completion function sample copied from the
Vim manual. This function determined `a:findstart` using a match against
`\a`, which only matches alphanumeric characters.

Git commit SHAs are SHA-1s, which can contain alphanumeric characters.
With the previous match regex, if the word to be completed contained
numbers, the `a:findstart` match would fail. This resulted in `a:base`
being empty, causing all SHAs to be added to the `matches` array.

Now we correctly match only the characters that can appear in a SHA-1,
and the completion correctly fills in the rest given the start of a SHA.

Only show the first 10 characters of the commit SHA in the popup menu to
allow more room for the subject. The first part of the SHA is enough to
be able to identify it, so this gets rid of some non-essential
information.

The full commit SHA will still be inserted as a result of the
completion.

Fix the subject issue from 0773fdd7bde5e7ce462c15bd1c71db3a3e46862a and
show the entire commit subject in the popup menu.

Initial try at adding the commit subject to the popup menu to provide
context for the commit.

Currently only shows the first word of the subject because I'm using
`split()`.

Don't force our functions to be loaded when the ftplugin is loaded. Be
good Vimscript citizens and respect load performance.