build/git-version: Only call 'git' once (#951)

Simplify the logic from 04848d1 (Add release build process,
2016-05-27, #47) to:

* Move from Bash to a generic POSIX shell.  This makes the script more
  portable (it will work on POSIX systems without Bash).  It does mean
  we need to drop the 'set' (which is not in POSIX [1]), but that's
  not a major problem because:

  * We can use &&-chaining instead of 'set -e'.
  * We do only have the one locally-defined variable, so we don't need
    'set -u'.
  * The simpler logic has no pipes, so 'set -o pipefail' would no
    longer matter.

* Replace the previous multiple Git calls with a single call to 'git
  describe'.

  * Use --dirty so we can drop the previous diff call.

  * Drop --tags, because we don't want to use lightweight
    (non-annotated) tags.  We want to use annotated tags, because we
    want to use signed release tags.  There's no code check for tag
    signatures here, but RELEASING.md requests 'tag -s vX.Y.Z', which
    will create signed, annotated tags.

  * Git's SHA-1 hashes are 40 hex digits, but I've used --abbrev=100
    to get unabbreviated hashes even in the face of future hash
    transitions [2].  This is a bit of a hack compared to the previous
    approach, because at some point Git may have >100-char hashes.
    But if that happens, it will be far enough in the future that I
    think the simplification we get here is worth it (and this
    constant is easy to bump if we need to).

  * The new call always returns the most recent tag, and (for commits
    which are not exact tag matches) also includes the abbreviated
    commit.  To isolate the hash (and possible dirty-mark) for commits
    that are not exact tags, I use POSIX's ${parameter##[word]}
    prefix-pattern removal [3].  Exact tags are unlikely to include
    '-g' (unless it is in the tag itself), so for them the
    prefix-removal is a no-op.  Again, this is a bit of a hack, since
    a tag might include '-g', but I think that's unlikely enough (as
    mentioned above, RELEASING.md requests vX.Y.Z for tags) that the
    simplification we get here is worth it.

[1]: http://pubs.opengroup.org/onlinepubs/9699919799/idx/utilities.html
[2]: https://github.com/git/git/blob/v2.16.3/Documentation/technical/hash-function-transition.txt
[3]: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_06_02

Author: wking

build/git-version.sh

@@ -1,22 +1,4 @@
-#!/usr/bin/env bash
-set -euo pipefail
+#!/bin/sh
 
-# parse the current git commit hash
-COMMIT=$(git rev-parse HEAD)
-
-# check if the current commit has a matching tag
-TAG=$(git describe --exact-match --abbrev=0 --tags ${COMMIT} 2> /dev/null || true)
-
-# use the matching tag as the version, if available
-if [ -z "$TAG" ]; then
-    VERSION=$COMMIT
-else
-    VERSION=$TAG
-fi
-
-# check for changed files (not untracked files)
-if [ -n "$(git diff --shortstat 2> /dev/null | tail -n1)" ]; then
-    VERSION="${VERSION}-dirty"
-fi
-
-echo $VERSION
+DESCRIPTION=$(git describe --abbrev=100 --dirty) &&
+echo "${DESCRIPTION##*-g}"