mingw: respect core.longPaths when initializing a new gitdir

Use case:
Allow git to checkout a repository or submodule in a directory with a long
path when core.longpaths = true.

Example:
> ./git.exe config --global core.longpaths true
> ./git.exe clone https://github.com/git/git.git --recurse-submodules \
/c/eval/git_test/loooooooooooooooooooooooooooooooooooooooooooooooooooooooo\
oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo\
oooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooo\
oooooooong

Context:
$ sh --version
	GNU bash, version 4.4.23(1)-release (x86_64-pc-msys)
$ ./git.exe --version --build-options
	git version 2.36.1.windows.1
	cpu: x86_64
	built from commit: e2ff68a
	sizeof-long: 4
	sizeof-size_t: 8
	shell-path: /bin/sh
	feature: fsmonitor--daemon

Error:
fatal: '$GIT_DIR' too big.

Problem analysis:
setup_explicit_git_dir in setup.c uses PATH_MAX to check if the git dir is
to long. On Windows PATH_MAX is set by limit.h to 260 and
setup_explicit_git_dir ignores core.longpaths.

Solution:
The implementation is based on the solution proposed by Johannes
Schindelin, see:
#3372 (comment)

* Refactor the part of trace2_initialize() that reads the config.
* Make tr2_sysenv_cb() a public function.
* No longer calling it from trace2_initialize(), but from a static callback
  function in common-main.c.
* Calling read_very_early_config() explicitly in main(), with that static
  callback function that calls into tr2_sysenv_cb().
* Extend the static callback function for Windows, and parse core.longPaths
  in that function.
* Extend startup_info struct in cache.h with 'int max_long_path' so we can
  use it in setup_explicit_git_dir instead of PATH_MAX.

This fixes #3372

Signed-off-by: Kevin Worm <kevin.worm_1@signify.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: kevin-worm

common-main.c

@@ -27,7 +27,10 @@ static void restore_sigpipe_to_default(void)
 
 static int read_very_early_config_cb(const char *key, const char *value, void *d)
 {
-	return tr2_sysenv_cb(key, value, d);
+	if (starts_with(key, "core."))
+		return platform_core_config(key, value, d);
+	else
+		return tr2_sysenv_cb(key, value, d);
 }
 
 int main(int argc, const char **argv)

compat/mingw.h

@@ -552,6 +552,12 @@ int is_valid_win32_path(const char *path, int allow_literal_nul);
  */
 int handle_long_path(wchar_t *path, int len, int max_path, int expand);
 
+static inline int mingw_get_max_path_size(void)
+{
+	return core_long_paths ? MAX_LONG_PATH : MAX_PATH;
+}
+#define get_max_path_size mingw_get_max_path_size
+
 /**
  * Converts UTF-8 encoded string to UTF-16LE.
  *

git-compat-util.h

@@ -372,6 +372,10 @@ char *gitdirname(char *);
 #define PATH_MAX 4096
 #endif
 
+#ifndef get_max_path_size
+#define get_max_path_size() PATH_MAX
+#endif
+
 typedef uintmax_t timestamp_t;
 #define PRItime PRIuMAX
 #define parse_timestamp strtoumax

setup.c

@@ -875,7 +875,7 @@ static const char *setup_explicit_git_dir(const char *gitdirenv,
 	char *gitfile;
 	int offset;
 
-	if (PATH_MAX - 40 < strlen(gitdirenv))
+	if (get_max_path_size() - 40 < strlen(gitdirenv))
 		die(_("'$%s' too big"), GIT_DIR_ENVIRONMENT);
 
 	gitfile = (char*)read_gitfile(gitdirenv);