duplicates:

• SR-3275 Lack of command 'git' causes swift-build crash.

Issue Description:

When swift build attempts to build on a system where the `git` executable is not installed, a fatal error occurs:

$ swift build
```
Cloning https://example.com/example.git
git clone --recursive --depth 10 https://example.com/example.git /tmp/example/Packages/example.git

fatal error: unexpectedly found nil while unwrapping an Optional value
Current stack trace:
0 libswiftCore.so 0x00007f269b2888e0 swift_reportError + 117
1 libswiftCore.so 0x00007f269b299e00 _swift_stdlib_reportFatalError + 61
2 libswiftCore.so 0x00007f269b0a58a3 <unavailable> + 0
3 libswiftCore.so 0x00007f269b202ffd <unavailable> + 0
4 libswiftCore.so 0x00007f269b0a58a3 <unavailable> + 0
5 libswiftCore.so 0x00007f269b1c48b0 specialized _fatalErrorMessage(StaticString, StaticString, StaticString, UInt, flags : UInt32) -> Never + 96
6 swift-build 0x00000000004933a5 <unavailable> + 0
7 swift-build 0x0000000000650007 <unavailable> + 0
8 swift-build 0x000000000065ee5d <unavailable> + 0
9 swift-build 0x0000000000660b62 <unavailable> + 0
10 swift-build 0x0000000000655d11 <unavailable> + 0
11 swift-build 0x0000000000657c8e <unavailable> + 0
12 swift-build 0x00000000006573ad <unavailable> + 0
13 swift-build 0x0000000000657d5c <unavailable> + 0
14 libswiftCore.so 0x00007f269b1470a0 Sequence.flatMap<A where ...> ((A.Iterator.Element) throws -> A1) throws -> [A1.Iterator.Element] + 576
15 swift-build 0x0000000000653fac <unavailable> + 0
16 swift-build 0x0000000000653984 <unavailable> + 0
17 swift-build 0x000000000065c002 <unavailable> + 0
18 swift-build 0x00000000006cd67b <unavailable> + 0
19 swift-build 0x00000000006fd335 <unavailable> + 0
20 swift-build 0x00000000004166d8 <unavailable> + 0
21 libc.so.6 0x00007f269925f740 __libc_start_main + 240
22 swift-build 0x0000000000416599 <unavailable> + 0
Illegal instruction

Swift 3.0.1 on Ubuntu 16.04