coverity: allow overriding the Coverity project

coverity: allow overriding the Coverity project

By default, the builds are submitted to the `git` project at
https://scan.coverity.com/projects/git.

The Git for Windows project would like to use this workflow, too,
though, and needs the builds to be submitted to the `git-for-windows`
Coverity project.

To that end, allow configuring the Coverity project name via the
repository variable, you guessed it, `COVERITY_PROJECT`. The default if
that variable is not configured or has an empty value is still `git`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>