Merge branch 'jh/trace2-sid-fix'

Polishing of the new trace2 facility continues.  The system-level
configuration can specify site-wide trace2 settings, which can be
overridden with per-user configuration and environment variables.

* jh/trace2-sid-fix:
  trace2: fixup access problem on /etc/gitconfig in read_very_early_config
  trace2: update docs to describe system/global config settings
  trace2: make SIDs more unique
  trace2: clarify UTC datetime formatting
  trace2: report peak memory usage of the process
  trace2: use system/global config for default trace2 settings
  config: add read_very_early_config()
  trace2: find exec-dir before trace2 initialization
  trace2: add absolute elapsed time to start event
  trace2: refactor setting process starting time
  config: initialize opts structure in repo_read_config()

Author: gitster

Documentation/config.txt

@@ -422,6 +422,8 @@ include::config/submodule.txt[]
 
 include::config/tag.txt[]
 
+include::config/trace2.txt[]
+
 include::config/transfer.txt[]
 
 include::config/uploadarchive.txt[]

Documentation/config/trace2.txt

@@ -0,0 +1,56 @@
+Trace2 config settings are only read from the system and global
+config files; repository local and worktree config files and `-c`
+command line arguments are not respected.
+
+trace2.normalTarget::
+	This variable controls the normal target destination.
+	It may be overridden by the `GIT_TR2` environment variable.
+	The following table shows possible values.
+
+trace2.perfTarget::
+	This variable controls the performance target destination.
+	It may be overridden by the `GIT_TR2_PERF` environment variable.
+	The following table shows possible values.
+
+trace2.eventTarget::
+	This variable controls the event target destination.
+	It may be overridden by the `GIT_TR2_EVENT` environment variable.
+	The following table shows possible values.
++
+include::../trace2-target-values.txt[]
+
+trace2.normalBrief::
+	Boolean.  When true `time`, `filename`, and `line` fields are
+	omitted from normal output.  May be overridden by the
+	`GIT_TR2_BRIEF` environment variable.  Defaults to false.
+
+trace2.perfBrief::
+	Boolean.  When true `time`, `filename`, and `line` fields are
+	omitted from PERF output.  May be overridden by the
+	`GIT_TR2_PERF_BRIEF` environment variable.  Defaults to false.
+
+trace2.eventBrief::
+	Boolean.  When true `time`, `filename`, and `line` fields are
+	omitted from event output.  May be overridden by the
+	`GIT_TR2_EVENT_BRIEF` environment variable.  Defaults to false.
+
+trace2.eventNesting::
+	Integer.  Specifies desired depth of nested regions in the
+	event output.  Regions deeper than this value will be
+	omitted.  May be overridden by the `GIT_TR2_EVENT_NESTING`
+	environment variable.  Defaults to 2.
+
+trace2.configParams::
+	A comma-separated list of patterns of "important" config
+	settings that should be recorded in the trace2 output.
+	For example, `core.*,remote.*.url` would cause the trace2
+	output to contain events listing each configured remote.
+	May be overridden by the `GIT_TR2_CONFIG_PARAMS` environment
+	variable.  Unset by default.
+
+trace2.destinationDebug::
+	Boolean.  When true Git will print error messages when a
+	trace target destination cannot be opened for writing.
+	By default, these errors are suppressed and tracing is
+	silently disabled.  May be overridden by the
+	`GIT_TR2_DST_DEBUG` environment variable.