remove all sources of uppercase SHA1 values

[chrisd8088]

I would be ideal to eliminate the need to down-case SHA1 values in
GitObjects and GitRepo, which could be achieved if we can ensure all sources of such SHA strings are always lowercase.

At least one such source is presumably in Sha1Id, but these may be persisted (e.g., in databases or files) and may also arrive from other sources.

If eliminating all sources of uppercase SHAs is difficult or impossible, then be current down-case-on-demand approach is sufficient, but an investigation would be desirable before concluding that permanently.