Auto-generate git versions compatible with PEP 440

`git archive` is configured to set osc version according to the git tag
via .gitattributes/export-subst

.gitattributes

@@ -0,0 +1,2 @@
+osc/util/git_version.py export-subst
+

osc/core.py

@@ -5,7 +5,10 @@
 
 from __future__ import print_function
 
-__version__ = '0.179'
+
+from .util import git_version
+__version__ = git_version.get_version('0.179.0')
+
 
 # __store_version__ is to be incremented when the format of the working copy
 # "store" changes in an incompatible way. Please add any needed migration

osc/util/git_version.py

@@ -0,0 +1,75 @@
+import os
+import subprocess
+
+
+def get_git_archive_version():
+    """
+    Return version that is set by git during `git archive`.
+    The returned format is equal to what `git describe --tags` returns.
+    """
+    # the `version` variable contents get substituted during `git archive`
+    # it requires adding this to .gitattributes: <path to this file> export-subst
+    version = "$Format:%(describe:tags=true)$"
+    if version.startswith("$"):
+        # version hasn't been substituted during `git archive`
+        return None
+    return version
+
+
+def get_git_version():
+    """
+    Determine version from git repo by calling `git describe --tags`.
+    """
+    cmd = ["git", "describe", "--tags"]
+    # run the command from the place where this file is placed
+    # to ensure that we're in a git repo
+    cwd = os.path.dirname(__file__)
+    proc = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=subprocess.PIPE, cwd=cwd)
+    stdout, _ = proc.communicate()
+
+    if proc.returncode != 0:
+        return None
+
+    version = stdout.strip().decode("utf-8")
+    return version
+
+
+def get_version(version):
+    """
+    Get the most relevant version of the software:
+    1. the version set during `git archive`
+    2. the version from the git tags by calling `git describe --tags`
+    3. the version explicitly specified in the source code
+
+    The version conforms PEP 440.
+    """
+    # use version from the archive
+    git_version = get_git_archive_version()
+
+    # use version from the git repo
+    if not git_version:
+        git_version = get_git_version()
+
+    # unable to determine version from git
+    if not git_version:
+        return version
+
+    if "-" not in git_version:
+        git_tag = git_version
+        git_commits = None
+        git_hash = None
+    else:
+        git_tag, git_commits, git_hash = git_version.rsplit("-", 2)
+        git_commits = int(git_commits)
+        # remove the 'g' prefix from hash
+        git_hash = git_hash[1:]
+
+    if version and git_tag != version:
+        msg = "Git tag '{}' doesn't correspond with version '{}' specified in the source code".format(git_tag, version)
+        raise ValueError(msg)
+
+    result = git_tag
+    if git_hash:
+        result += "+{}.git.{}".format(git_commits, git_hash)
+
+    return result