Handle missing Git in git_remote_head_branch

[mortenpi]

This is currently against release-0.27 since I wrote this on top of the 0.27.20 tag, and needs to be brought up to date with with master.

Fix #1870.

[mortenpi]

@glennmoy Any chance you could run this branch in your Git-less Docker container, to see if it fixes the problem and to make sure that there are no more oversights.

[glennmoy]

@glennmoy Any chance you could run this branch in your Git-less Docker container, to see if it fixes the problem and to make sure that there are no more oversights.

yes that works! thank you!

julia> doctest(MyPackage)
┌ Warning: Unable to determine HTML(edit_link = ...) from remote HEAD branch, defaulting to "master".
│ Unable to find the `git` binary. Unless this is due to a configuration error, the
│ relevant variable should be set explicitly.
└ @ Documenter.Utilities ~/.julia/packages/Documenter/scBHi/src/Utilities/Utilities.jl:797
[ Info: SetupBuildDirectory: setting up build directory.
[ Info: Doctest: running doctests.
[ Info: Skipped ExpandTemplates step (doctest only).
[ Info: Skipped CrossReferences step (doctest only).
[ Info: Skipped CheckDocument step (doctest only).
[ Info: Skipped Populate step (doctest only).
[ Info: Skipped RenderDocument step (doctest only).
Test Summary:     | Pass  Total
Doctests: MyPackage |    1      1
Test.DefaultTestSet("Doctests: MyPackage", Any[], 1, false, false)