Generalize upgrade-makefile target to support other files in REPRO distribution

[tmcphillips]

• Add configuration option setting upstream repository for upgrades.
• Make repro-templates update from repro, and children of repro-templates from repro-templates by default.
• Add new target upgrade-repro that upgrades all supported files.
• Save old versions of each file in a .repro-backup directory.
• Support upgrading Makfile-tests.