GIT-VERSION-GEN: fix overriding version via environment

GIT-VERSION-GEN tries to derive the version that Git is being built from
via multiple different sources in the following order:

  1. A file called "version" in the source tree's root directory, if it
     exists.

  2. The current commit in case Git is built from a Git repository.

  3. Otherwise, we use a fallback version stored in a variable which is
     bumped whenever a new Git version is getting tagged.

It used to be possible to override the version by overriding the
`GIT_VERSION` Makefile variable (e.g. `make GIT_VERSION=foo`). This
worked somewhat by chance, only: `GIT-VERSION-GEN` would write the
actual Git version into `GIT-VERSION-FILE`, not the overridden value,
but when including the file into our Makefile we would not override the
`GIT_VERSION` variable because it has already been set by the user. And
because our Makefile used the variable to propagate the version to our
build tools instead of using `GIT-VERSION-FILE` the resulting build
artifacts used the overridden version.

But that subtle mechanism broke with 4838dea (Makefile: refactor
GIT-VERSION-GEN to be reusable, 2024-12-06) and subsequent commits
because the version information is not propagated via the Makefile
variable anymore, but instead via the files that `GIT-VERSION-GEN`
started to write. And as the script never knew about the `GIT_VERSION`
environment variable in the first place it uses one of the values listed
above instead of the overridden value.

Fix this issue by making `GIT-VERSION-GEN` handle the case where
`GIT_VERSION` has been set via the environment.

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

Author: pks-t

GIT-VERSION-GEN

@@ -29,7 +29,10 @@ export GIT_CEILING_DIRECTORIES
 
 # First see if there is a version file (included in release tarballs),
 # then try git-describe, then default.
-if test -f "$SOURCE_DIR"/version
+if test -n "$GIT_VERSION"
+then
+    VN="$GIT_VERSION"
+elif test -f "$SOURCE_DIR"/version
 then
 	VN=$(cat "$SOURCE_DIR"/version) || VN="$DEF_VER"
 elif {