GIT-VERSION-GEN: simplify computing the dirty marker

The GIT-VERSION-GEN script computes the version that Git is being built
from. When building from a commit with an unclean worktree it knows to
append "-dirty" to that version to indicate that there were custom
changes applied and that it isn't the exact same as that commit.

The dirtiness check is done manually via git-diff-index(1), which is
somewhat puzzling though: we already use git-describe(1) to compute the
version, which also knows to compute dirtiness via the "--dirty" flag.
But digging back in history explains why: the "-dirty" suffix was added
in 31e0b2c (GIT 1.5.4.3, 2008-02-23), and git-describe(1) didn't yet
have support for "--dirty" back then.

Refactor the script to use git-describe(1). Despite being simpler, it
also results in a small speedup:

    Benchmark 1: git describe --dirty --match "v[0-9]*"
      Time (mean ± σ):      12.5 ms ±   0.3 ms    [User: 6.3 ms, System: 8.8 ms]
      Range (min … max):    12.0 ms …  13.5 ms    200 runs

    Benchmark 2: git describe --match "v[0-9]*" HEAD && git update-index -q --refresh && git diff-index --name-only HEAD --
      Time (mean ± σ):      17.9 ms ±   1.1 ms    [User: 8.8 ms, System: 14.4 ms]
      Range (min … max):    17.0 ms …  30.6 ms    148 runs

    Summary
      git describe --dirty --match "v[0-9]*" ran
        1.43 ± 0.09 times faster than git describe --match "v[0-9]*" && git update-index -q --refresh && git diff-index --name-only HEAD --

While the speedup doesn't really matter on Unix-based systems, where
filesystem operations are typically fast, they do matter on Windows
where the commands take a couple hundred milliseconds. A quick and dirty
check on that system shows a speedup from ~800ms to ~400ms.

Signed-off-by: Patrick Steinhardt <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: pks-t

GIT-VERSION-GEN

@@ -39,13 +39,9 @@ then
 			test -d "${GIT_DIR:-.git}" ||
 			test -f "$SOURCE_DIR"/.git;
 		} &&
-		VN=$(git -C "$SOURCE_DIR" describe --match "v[0-9]*" HEAD 2>/dev/null) &&
+		VN=$(git -C "$SOURCE_DIR" describe --dirty --match="v[0-9]*" 2>/dev/null) &&
 		case "$VN" in
 		*$LF*) (exit 1) ;;
-		v[0-9]*)
-			git -C "$SOURCE_DIR" update-index -q --refresh
-			test -z "$(git -C "$SOURCE_DIR" diff-index --name-only HEAD --)" ||
-			VN="$VN-dirty" ;;
 		esac
 	then
 		VN=$(echo "$VN" | sed -e 's/-/./g');