manifest: support optional --manifest-name
manifest: support optional --manifest-name

Still use the repo manifest by default as before, but gives us
the option of overriding it to support e.g.: using a subset of
the full manifest.

Change-Id: Ia42cd1cb3a0a58929d31bb01c9724e9d31f68730
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/256372
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>
Tested-by: Sean McAllister <ed4d2242d1138bb5c12488824add2e57e1144bd4@google.com>

aosp-mirror/tools_repo,lewixliu/git-repo,GerritCodeReview/git-repo,TheQtCompany/git-repo,lewixliu/git-repo,couchbasedeps/git-repo,codetutil/git-repo,codetutil/git-repo,lipro-yocto/git-repo,mer-tools/git-repo,lipro-yocto/git-repo,mer-tools/git-repo,aosp-mirror/tools_repo,TheQtCompany/git-repo,GerritCodeReview/git-repo,couchbasedeps/git-repo