cmd/gitmirror: skip some tests that require git when git isn't available
cmd/gitmirror: skip some tests that require git when git isn't available

More work on getting x/build to pass.

Change-Id: Ie70919f597b72daafccc2550a106be2421c8d7ba
Reviewed-on: https://go-review.googlesource.com/c/157442
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

golang/build,golang/build,golang/build,golang/build