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.
remove code below asserts in limitedmap.h (fixes a ToDo)
[Qt] minor changes in splashscreen.cpp

- guard an unused variable with Q_UNUSED() macro
- remove a commented out line of code
Fixing out of bounds asses error
Avoiding referencing elements of an empty vector
Reinitializing list's begin iterator after few elements were erased from the head
Fixed typo in README.md file. "-regest" "-regtest"

There was a typo in the README.md file. It previously read "-regest" instead of the proper "-regtest" and it has been updated to the proper reading.
Merge pull request #4974

b498a94 Fixed typo in README.md file. "-regest" "-regtest" (Eric Shaw)
[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
Merge pull request #4782

2e5361b remove code below asserts in limitedmap.h (fixes a ToDo) (Philip Kaufmann)
Merge pull request #4967

459a2d2 Avoiding referencing elements of an empty vector (ENikS)
Merge pull request #4959

bb26e2c [Qt] minor changes in splashscreen.cpp (Philip Kaufmann)
script: Fix reference into empty vector run time exception

Edit by laanwj: `begin_ptr(sourcedata) + sourcedata.size()` -> `end_ptr(sourcedata)`
Merge pull request #4968

219372f script: Fix reference into empty vector run time exception (ENikS)