Improve consistency of git-var

This patch series makes a few distinct improvements to git-var to support the change to git_editor() prompted [here][1] and ultimately support that patch to introduce GIT_SEQUENCE_EDITOR as a handled logical variable.

Changes since v2:

* Nix premature assignment of git_var and val, preferring to let the compiler tell us when they're being used before init.
* Factor out sane_unset_all_editors for tests to reduce duplication
* Use more specific test_* helper functions

Sean Allred (2):
var: do not print usage() with a correct invocation
var: allow GIT_EDITOR to return null

 Documentation/git-var.txt |  3 +-
 builtin/var.c             | 29 +++++++++---------
 t/t0007-git-var.sh        | 62 +++++++++++++++++++++++++++++++++++++++
 3 files changed, 78 insertions(+), 16 deletions(-)

base-commit: c000d916380bb59db69c78546928eadd076b9c7d
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1434%2Fvermiculus%2Fsa%2Fvar-improvements-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1434/vermiculus/sa/var-improvements-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1434

Range-diff vs v2:

1: a7ff842a3e8 ! 1: 889fdf877a1 var: do not print usage() with a correct invocation

     @@ builtin/var.c: static int show_config(const char *var, const char *value, void *
       
       int cmd_var(int argc, const char **argv, const char *prefix)
       {
     -+	const struct git_var *git_var = NULL;
     - 	const char *val = NULL;
     +-	const char *val = NULL;
     ++	const struct git_var *git_var;
     ++	const char *val;
     ++
       	if (argc != 2)
       		usage(var_usage);
     + 
      @@ builtin/var.c: int cmd_var(int argc, const char **argv, const char *prefix)
       		return 0;
       	}
 2:  427cb7b55ac ! 2:  3d8bf3662fe var: allow GIT_EDITOR to return null
     @@ builtin/var.c: static const char var_usage[] = "git var (-l | <variable>)";
       static const char *pager(int flag)
      
       ## t/t0007-git-var.sh ##
     +@@ t/t0007-git-var.sh: test_description='basic sanity checks for git var'
     + TEST_PASSES_SANITIZE_LEAK=true
     + . ./test-lib.sh
     + 
     ++sane_unset_all_editors () {
     ++	sane_unset GIT_EDITOR &&
     ++	sane_unset VISUAL &&
     ++	sane_unset EDITOR
     ++}
     ++
     + test_expect_success 'get GIT_AUTHOR_IDENT' '
     + 	test_tick &&
     + 	echo "$GIT_AUTHOR_NAME <$GIT_AUTHOR_EMAIL> $GIT_AUTHOR_DATE" >expect &&
      @@ t/t0007-git-var.sh: test_expect_success 'get GIT_DEFAULT_BRANCH with configuration' '
       	)
       '
       
      +test_expect_success 'get GIT_EDITOR without configuration' '
      +	(
     -+		sane_unset GIT_EDITOR &&
     -+		sane_unset VISUAL &&
     -+		sane_unset EDITOR &&
     -+		>expect &&
     -+		! git var GIT_EDITOR >actual &&
     -+		test_cmp expect actual
     ++		sane_unset_all_editors &&
     ++		test_expect_code 1 git var GIT_EDITOR >out &&
     ++		test_must_be_empty out
      +	)
      +'
      +
      +test_expect_success 'get GIT_EDITOR with configuration' '
      +	test_config core.editor foo &&
      +	(
     -+		sane_unset GIT_EDITOR &&
     -+		sane_unset VISUAL &&
     -+		sane_unset EDITOR &&
     ++		sane_unset_all_editors &&
      +		echo foo >expect &&
      +		git var GIT_EDITOR >actual &&
      +		test_cmp expect actual
     @@ t/t0007-git-var.sh: test_expect_success 'get GIT_DEFAULT_BRANCH with configurati
      +
      +test_expect_success 'get GIT_EDITOR with environment variable GIT_EDITOR' '
      +	(
     -+		sane_unset GIT_EDITOR &&
     -+		sane_unset VISUAL &&
     -+		sane_unset EDITOR &&
     ++		sane_unset_all_editors &&
      +		echo bar >expect &&
      +		GIT_EDITOR=bar git var GIT_EDITOR >actual &&
      +		test_cmp expect actual
     @@ t/t0007-git-var.sh: test_expect_success 'get GIT_DEFAULT_BRANCH with configurati
      +
      +test_expect_success 'get GIT_EDITOR with environment variable EDITOR' '
      +	(
     -+		sane_unset GIT_EDITOR &&
     -+		sane_unset VISUAL &&
     -+		sane_unset EDITOR &&
     ++		sane_unset_all_editors &&
      +		echo bar >expect &&
      +		EDITOR=bar git var GIT_EDITOR >actual &&
      +		test_cmp expect actual
     @@ t/t0007-git-var.sh: test_expect_success 'get GIT_DEFAULT_BRANCH with configurati
      +test_expect_success 'get GIT_EDITOR with configuration and environment variable GIT_EDITOR' '
      +	test_config core.editor foo &&
      +	(
     -+		sane_unset GIT_EDITOR &&
     -+		sane_unset VISUAL &&
     -+		sane_unset EDITOR &&
     ++		sane_unset_all_editors &&
      +		echo bar >expect &&
      +		GIT_EDITOR=bar git var GIT_EDITOR >actual &&
      +		test_cmp expect actual
     @@ t/t0007-git-var.sh: test_expect_success 'get GIT_DEFAULT_BRANCH with configurati
      +test_expect_success 'get GIT_EDITOR with configuration and environment variable EDITOR' '
      +	test_config core.editor foo &&
      +	(
     -+		sane_unset GIT_EDITOR &&
     -+		sane_unset VISUAL &&
     -+		sane_unset EDITOR &&
     ++		sane_unset_all_editors &&
      +		echo foo >expect &&
      +		EDITOR=bar git var GIT_EDITOR >actual &&
      +		test_cmp expect actual

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