git-gui: Unabbreviate commit SHA-1s prior to display

git-gui: Unabbreviate commit SHA-1s prior to display

If the end-user feeds us an abbreviated SHA-1 on the command line for
`git gui browser` or `git gui blame` we now unabbreviate the value
through `git rev-parse` so that the title section of the blame or
browser window shows the user the complete SHA-1 as Git determined
it to be.

If the abbreviated value was ambiguous we now complain with the
standard error message(s) as reported by git-rev-parse --verify,
so that the user can understand what might be wrong and correct
their command line.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>