fix version when installing from git archive

When installing from a git archive downloaded from github, the file
scapy/VERSION is not present since it is generated when building a
source archive with:

    ./setup.py sdist

To work around the problem, use the export-subst git attribute to make
git archive write the current revison information in scapy/__init__.py.

Signed-off-by: Robin Jarry <robin.jarry@6wind.com>

.gitattributes

@@ -0,0 +1 @@
+scapy/__init__.py	export-subst

scapy/__init__.py

@@ -71,7 +71,17 @@ def _version():
                 tag = f.read()
             return tag
         except:
-            return 'unknown.version'
+            # Rely on git archive "export-subst" git attribute.
+            # See 'man gitattributes' for more details.
+            git_archive_id = '$Format:%h %d$'
+            sha1 = git_archive_id.strip().split()[0]
+            match = re.search(r'tag:(\S+)', git_archive_id)
+            if match:
+                return match.group(1)
+            elif sha1:
+                return sha1
+            else:
+                return 'unknown.version'
 
 VERSION = _version()