Use --reference-if-able in bootstrap

[ndevenish]

cctbx_project has added submodules now. This caused errors when the submodule didn't have a reference copy on the DLS filesystem. This wasn't picked up by Jenkins because the repository is checked out separately from bootstrap (by Jenkins) on our daily jobs.

cctbx_project: ERROR - Cannot checkout git repository
                       Cloning into 'cctbx_project'...
Updating files: 100% (6477/6477), done. 87% (5676/6477)
                       Submodule 'xfel/euxfel/definitions' (https://github.com/HDRMX/definitions) registered for path 'xfel/euxfel/definitions'
                       hint: An alternate computed from a superproject's alternate is invalid.
                       hint: To allow Git to clone without an alternate in such a case, set
                       hint: submodule.alternateErrorStrategy to 'info' or, equivalently, clone with
                       hint: '--reference-if-able' instead of '--reference'.
                       fatal: submodule 'xfel/euxfel/definitions' cannot add alternate: path '/dls/science/groups/scisoft/DIALS/repositories/git-reference/cctbx_project/.git/modules/xfel/euxfel/definitions/' does not exist
                       Failed to clone 'xfel/euxfel/definitions'. Retry scheduled
                       hint: An alternate computed from a superproject's alternate is invalid.
                       hint: To allow Git to clone without an alternate in such a case, set
                       hint: submodule.alternateErrorStrategy to 'info' or, equivalently, clone with
                       hint: '--reference-if-able' instead of '--reference'.
                       fatal: submodule 'xfel/euxfel/definitions' cannot add alternate: path '/dls/science/groups/scisoft/DIALS/repositories/git-reference/cctbx_project/.git/modules/xfel/euxfel/definitions/' does not exist
                       Failed to clone 'xfel/euxfel/definitions' a second time, aborting

This does make git 2.11 a hard dependency on systems where the reference logic is triggered.

[ndevenish]

CentOS 7 appears to have an un-updated default of git-1.8.3.1, darn.

[ndevenish]

Okay, this isn't perfect - it'll still fail on old git versions if the reference isn't up-to-date, but won't fail in other cases for old git versions.

[ndevenish]

git version 2.29.2.windows.2 obviously