ICU-20229 update make dist to use git, not svn

ICU-20229 update make dist to use git, not svn

- use git instead of svn for versioning
- update generated file names to match official downloads

Example filenames:
- icu4c-63_1-c5749aaf94-src.tgz (includes git hash or tag)
- icu4c-63_1-src.tgz (hard link to above)
- icu4c-src.tgz (symlink)

- icu4c-63_1-c5749aaf94-src.zip (includes git hash or tag)
- icu4c-63_1-src.zip (hard link to above)
- icu4c-src.zip (symlink)

- icu4c-63_1-c5749aaf94-data.zip (includes git hash or tag)
- icu4c-63_1-data.zip (hard link to above)
- icu4c-data.zip (symlink)

- icu4c-63_1-c5749aaf94-docs.zip (includes git hash or tag)
- icu4c-63_1-docs.zip (hard link to above)
- icu4c-docs.zip (symlink)