install pygit2: fatal error: 'git2.h' file not found

maybe this is not a git-explode issue, but still I've run into it:

`sudo pip install git-deps`

results in:

```
creating build/temp.macosx-10.11-x86_64-2.7
building '_pygit2' extension
creating build/temp.macosx-10.11-x86_64-2.7/src
clang -fno-strict-aliasing -fno-common -dynamic -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -I/usr/local/include -I/usr/local/include -I/usr/local/opt/openssl/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python@2/2.7.15/Frameworks/Python.framework/Versions/2.7/include/python2.7 -c src/blob.c -o build/temp.macosx-10.11-x86_64-2.7/src/blob.o
In file included from src/blob.c:30:
src/blob.h:33:10: fatal error: 'git2.h' file not found
#include
^
1 error generated.
error: command 'clang' failed with exit status 1

----------------------------------------
Command "/usr/local/opt/python@2/bin/python2.7 -u -c "import setuptools, tokenize;__file__='/private/tmp/pip-install-u4aDlM/pygit2/setup.py';f=getattr(tokenize, 'open', open)(__file__);code=f.read().replace('\r\n', '\n');f.close();exec(compile(code, __file__, 'exec'))" install --record /private/tmp/pip-record-_osVGz/install-record.txt --single-version-externally-managed --compile" failed with error code 1 in /private/tmp/pip-install-u4aDlM/pygit2/
```

Inizia riproducendo `sudo pip install git-deps` nell’ambiente macOS Python 2.7 segnalato ed esaminando i requisiti di compilazione di pygit2 relativi all’header mancante `git2.h`. Verifica se il progetto documenta o controlla questa dipendenza; il lavoro è completo quando l’installazione riesce oppure quando i prerequisiti richiesti e il problema sono documentati chiaramente.

Scritto dal modello di indicizzazione a partire dal testo della issue.