Added git-clang-format as a pre-commit hook

[kjetilly]

I am opening this PR also to create a discussion. Before you go on reading, please note that this can be selectively disabled by exporting the SKIP environment variable to contain git-clang-format-fix=1, so eg.

SKIP=git-clang-format-fix=1 git commit -m 'Did stuff I do not want formatted'

As far as I can tell, there is no ready-made pre-commit hook for this, so a bash script is the solution for now.

Do note that this applies git-clang-format, so it will only clang-format the diff.

Example workflow where I have added extra spacing that should not be there:

$ git commit -m 'Made meaning more spatious'
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
git-clang-format (diff-only, auto-fix)...................................Failed
- hook id: git-clang-format-fix
- exit code: 1

diff --git a/opm/models/blackoil/blackoilprimaryvariables.hh b/opm/models/blackoil/blackoilprimaryvariables.hh
index 8aa1524d8..8030ceaf7 100644
--- a/opm/models/blackoil/blackoilprimaryvariables.hh
+++ b/opm/models/blackoil/blackoilprimaryvariables.hh
@@ -181,8 +181,8 @@ public:
         result.primaryVarsMeaningGas_ = GasMeaning::Rv;
         result.primaryVarsMeaningPressure_ = PressureMeaning::Pg;
         result.primaryVarsMeaningWater_ = WaterMeaning::Rsw;
-        result.primaryVarsMeaningSolvent_          = SolventMeaning::Ss;
-        for (std::size_t i = 0; i < result.size(); ++i)              {
+        result.primaryVarsMeaningSolvent_ = SolventMeaning::Ss;
+        for (std::size_t i = 0; i < result.size(); ++i) {
             result[i] = i + 1;
         }
 
[git-clang-format] Applying formatting to staged changes...
changed files:
    opm/models/blackoil/blackoilprimaryvariables.hh

[git-clang-format] Changes have been applied.
Please review and re-run your commit.


opm-simulators on  precommit [$!?] via  v3.31.6 
$ git commit -m 'Made meaning more spatious''
trim trailing whitespace.................................................Passed
fix end of files.........................................................Passed
git-clang-format (diff-only, auto-fix)...............(no files to check)Skipped
[precommit a4c90e0b7] added new precommit
 1 file changed, 2 insertions(+), 2 deletions(-)

Note that I have for the proof-of-concept kept the shell-script in the YAML-file. It is probably worthwhile to move this to a separate file, or even a separate repo if we want this for opm-common and grid as well, but before that I think it is wise to discuss wether this is something we want.