Installing nimterop head using nimble crashes
To reproduce this issue:

```
nimble install nimterop@#head
```

On running above, I get this crash:

```
 Installing nimterop@#head
   Building nimterop/nimterop/toast using c backend
       Tip: 73 messages have been suppressed, use --verbose to show them.
     Error: Build failed for package: nimterop
        ... Details:
        ... Execution failed with exit code 1
        ... Command: "/home/kmodi/usr_local/apps/6/nim/devel/bin/nim" c --noBabelPath -d:release --path:"/home/kmodi/.nimble/pkgs/regex-0.10.1"  --path:"/home/kmodi/.nimble/pkgs/unicodedb-0.6.0"  --path:"/home/kmodi/.nimble/pkgs/unicodeplus-0.4.0"  --path:"/home/kmodi/.nimble/pkgs/unicodedb-0.6.0"  --path:"/home/kmodi/.nimble/pkgs/cligen-#HEAD"  -o:"/tmp/nimble_1073/githubcom_genotrancenimterop_#head/nimterop/toast" "/tmp/nimble_1073/githubcom_genotrancenimterop_#head/nimterop/toast.nim"
        ... Output: Hint: used config file '/home/kmodi/usr_local/apps/6/nim/devel/config/nim.cfg' [Conf]
        ... Hint: used config file '/home/kmodi/usr_local/apps/6/nim/devel/config/config.nims' [Conf]
        ... Hint: system [Processing]
        ... Hint: widestrs [Processing]
        ... Hint: io [Processing]
        ... Hint: toast [Processing]
        ... Hint: os [Processing]
        ... Hint: strutils [Processing]
        ... Hint: parseutils [Processing]
        ... Hint: math [Processing]
        ... Hint: bitops [Processing]
        ... Hint: macros [Processing]
        ... Hint: algorithm [Processing]
        ... Hint: unicode [Processing]
        ... Hint: pathnorm [Processing]
        ... Hint: osseps [Processing]
        ... Hint: posix [Processing]
        ... Hint: times [Processing]
        ... Hint: options [Processing]
        ... Hint: typetraits [Processing]
        ... Hint: strformat [Processing]
        ... Hint: api [Processing]
        ... Hint: setup [Processing]
        ... Hint: git [Processing]
        ... Hint: osproc [Processing]
        ... Hint: strtabs [Processing]
        ... Hint: hashes [Processing]
        ... Hint: streams [Processing]
        ... Hint: cpuinfo [Processing]
        ... Hint: linux [Processing]
        ... Hint: regex [Processing]
        ... Hint: sets [Processing]
        ... Hint: tables [Processing]
        ... Hint: properties [Processing]
        ... Hint: properties_data [Processing]
        ... Hint: types [Processing]
        ... Hint: types_data [Processing]
        ... Hint: unicodeplus [Processing]
        ... Hint: paths [Processing]
        ... Hint: compat [Processing]
        ... Hint: types [Processing]
        ... Hint: time_t [Processing]
        ... Setting up Git repo: https://github.com/tree-sitter/tree-sitter/
        ... stack trace: (most recent call last)
        ... treesitter/api.nim(8, 18) api
        ... setup.nim(6, 10)         treesitterSetup
        ... git.nim(106, 21)         gitPull
        ... git.nim(23, 11)          execAction
        ... ../../../../home/kmodi/usr_local/apps/6/nim/devel/lib/system/assertions.nim(27, 20) failedAssertImpl
        ... ../../../../home/kmodi/usr_local/apps/6/nim/devel/lib/system/assertions.nim(20, 11) raiseAssert
        ... ../../../../home/kmodi/usr_local/apps/6/nim/devel/lib/system/fatal.nim(37, 5) sysFatal
        ... /home/kmodi/usr_local/apps/6/nim/devel/lib/system/fatal.nim(37, 5) Error: unhandled exception: /tmp/nimble_1073/githubcom_genotrancenimterop_#head/nimterop/git.nim(23, 14) `false` Command failed: (1, false)
        ... ccmd: cd '/tmp/nimble_1073/githubcom_genotrancenimterop_#head/build/inc/treesitter' && git init .
        ... result:
        ... /bin/sh: line 0: cd: /tmp/nimble_1073/githubcom_genotrancenimterop_#head/build/inc/treesitter: No such file or directory
```

At first it looked like ` nimble` issue. But I tried `nimble install inim@#head` and `nimble install cligen@#head`, and they both installed fine.

---

## Nim version

```
Nim Compiler Version 0.19.9 [Linux: amd64]
Compiled at 2019-04-03
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: dc2986789b703300ac0680a10004f77453b146c3
active boot switches: -d:release
```