build: fix release name strings for gitian builds

When building from a distdir as gitian does, checking for the .git dir
is not reliable. Instead, ask git if we're in a repo.
Fixing out of bounds asses error
[Qt] add more NULL pointer checks in bitcoingui

- add missing checks for clientModel and optionsModel
- small cleanups for an #ifdef on Mac
- remove an unneeded else
Merge pull request #4958

e6beedc [Qt] add more NULL pointer checks in bitcoingui (Philip Kaufmann)
RPC getnetworkinfo: export local node's client sub-version string
Merge pull request #4914

ff36cbe RPC getnetworkinfo: export local node's client sub-version string (Jeff Garzik)
Merge pull request #4966

c8063b5 Fixing out of bounds asses error (ENikS)
Merge pull request #4946

c65cc8c build: fix release name strings for gitian builds (Cory Fields)
Fixed the broken `brew` shell command