Git mirror: Replace svn info for revision detection by git log+perl

[simon04]

From ​https://github.com/openstreetmap/josm/pull/5 by ​https://github.com/AlfonZ42:

I didn't like that revision detection (hacked long ago in ​https://github.com/openstreetmap/josm/commit/bcbc70ea2e5c51b5d27222fe69c7e963645ca263) was trying to access josm.openstreetmap.de (it made building impossible when offline) and revision number was not always correct (when working with older revisions).

I replaced svn info by git log + perl.
git log looks at the most recent commit message with git-svn-id, which is then processed by perl script that produces revision number and date, which are needed in build.xml.
In case perl is not found, the build doesn't fail, values fallback to UNKNOWN, as specified in build.xml.

[Don-vip]

In 6540/josm:

fix #9450 - Allow to build JOSM from a git mirror clone with ant -Dgit_revision=true

[simon04]

In 6545/josm:

see #9450 - JOSM build from a git mirror: remove need for perl script, automatically obtain revision from Git if SVN fails (removes need for git_revision=true)

[Don-vip]

Better :) Thanks !

[simon04]

Finally, using a git-svn-repository is fun. :)

[simon04]

In 6585/josm:

fix #9501 see #9450 - JOSM build failed with SVN

[Stereo]

I was wondering what 0c6e7542-c601-0410-84e7-c038aed88b3b means, and if the stability of that string can be relied on?