environment: add GIT_PREFIX to local_repo_env

The GIT_PREFIX variable is set based on our location within
the working tree. It should therefore be cleared whenever
GIT_WORK_TREE is cleared.

In practice, this doesn't cause any bugs, because none of
the sub-programs we invoke with local_repo_env cleared
actually care about GIT_PREFIX. But this is the right thing
to do, and future proofs us against that assumption changing.

While we're at it, let's define a GIT_PREFIX_ENVIRONMENT
macro; this avoids repetition of the string literal, which
can help catch any spelling mistakes in the code.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

cache.h

@@ -345,6 +345,7 @@ static inline enum object_type object_type(unsigned int mode)
 #define GIT_DIR_ENVIRONMENT "GIT_DIR"
 #define GIT_NAMESPACE_ENVIRONMENT "GIT_NAMESPACE"
 #define GIT_WORK_TREE_ENVIRONMENT "GIT_WORK_TREE"
+#define GIT_PREFIX_ENVIRONMENT "GIT_PREFIX"
 #define DEFAULT_GIT_DIR_ENVIRONMENT ".git"
 #define DB_ENVIRONMENT "GIT_OBJECT_DIRECTORY"
 #define INDEX_ENVIRONMENT "GIT_INDEX_FILE"

environment.c

@@ -88,6 +88,7 @@ const char * const local_repo_env[] = {
 	GRAFT_ENVIRONMENT,
 	INDEX_ENVIRONMENT,
 	NO_REPLACE_OBJECTS_ENVIRONMENT,
+	GIT_PREFIX_ENVIRONMENT,
 	NULL
 };
 

setup.c

@@ -768,9 +768,9 @@ const char *setup_git_directory_gently(int *nongit_ok)
 
 	prefix = setup_git_directory_gently_1(nongit_ok);
 	if (prefix)
-		setenv("GIT_PREFIX", prefix, 1);
+		setenv(GIT_PREFIX_ENVIRONMENT, prefix, 1);
 	else
-		setenv("GIT_PREFIX", "", 1);
+		setenv(GIT_PREFIX_ENVIRONMENT, "", 1);
 
 	if (startup_info) {
 		startup_info->have_repository = !nongit_ok || !*nongit_ok;