Lookup the whitespace attribute for each modified file and dispatch
custom checks as necessary.  Currently the only custom check is the
approximate tab-in-indent check for Git < 1.7.2.

Revert commits always have longer first lines than the commit they
revert.  Do not reject those that happen to go over the threshold.  It
is much simpler if the "git revert" command creates the commit without
error.

Chain the pre-commit, commit-msg, and prepare-commit-msg hooks to
locally configured scripts.  Interpret relative paths with respect to
the working directory where the hooks run (top of work tree).  This
allows project setup scripts to add project-specific checks for each of
these hooks.

At this time, a contributor won't be able to commit a change if KWStyle
is not installed on the computer. KWStyle is not a very common program,
and so is likely to have to be installed by hand. This increase the work
needed to be able to contribute to ITK.
With this change a warning is displayed if KWStyle is not installed
but the commit is not blocked.

Change-Id: I0719ee5ac6e048120504bbdc4dc022043c0f0ded

uncrustify (uncrustify.sourceforge.net) is applied to changed files prior to
commit.

This feature is off by default.  To enable this behavior, set

  git config hooks.uncrustify true

By default, the behavior of git-mergetool is used to review the changes
uncrustify makes before they are added to the commit.  For more information on
this behavior, see

  git help mergetool

KWStyle is run on the changed C++ files and the commit is aborted if the files
do not pass the test.  A file similar to the original is saved with a '*.kws'
extension so that line numbers referenced in the error message can be examined.

The test is off by default.  To enable this behavoir, set

  git config hooks.KWStyle true

Project specific uncrustify and KWStyle configuration files are set with 'git
config'.  For example,

  git config hooks.uncrustify.conf path/to/uncrustify.conf
  git config hooks.KWStyle.conf path/to/KWStyle.conf
  git config hooks.KWStyle.overwriteRulesConf path/to/overwrite.conf # optional

If the appropriate values have not been set, die() is called.  An
optional KWStyle overwrite rules file can also been configured.

The files on which to run the style checks must also be identified in the
repository's '.gitattributes'.  For example,

  *.h    hooks.style
  *.cpp  hooks.style

Or, to only enable a subset of style hooks,

  *.h    hooks.style=KWStyle
  *.cpp  hooks.style=KWStyle,uncrustify

Change-Id: Ia6b2d4136af3002eb0ec5d36f03c50df928917f4

If hooks.submodule is 'false' disable all checks.  If an individual
hooks.<module>.update is 'true' then accept any update for that module.
Leave these options out of the hints printed.  Developers that know what
they are doing will be able to find them by reading the hook source.

Merge commits can have long subject lines because they name other
branches.  Accept such messages as a special case for merge commits.

Add a Gerrit Change-Id header if hooks.GerritId is true.  Print help if
the option is not set and a remote URL looks like Gerrit.

This script uses bashisms like $() so we need a real bash.

Currently the hook is unused but later we will invoke it from the main
commit-msg hook.

Check for leading TABs in files marked with the whitespace=tab-in-indent
attribute instead of hard-coding a list of file extensions.

Check for a 'hooks.MaxObjectKiB' attribute to set the limit of specific
files or patterns.

Since Git does not automatically update submodule checkouts when
checking out a new version in the work tree, it is common to have
locally modified submodule references.  Therefore it is easy to stage
such modifications with other changes by accident, especially with
commands like "git add -u" or "git commit -a".  The result is almost
always wrong if the submodule change is not intended.

Prevent such mistakes by requiring an extra step to commit submodule
link updates with other changes.  When this case is detected, print a
message describing the situation and provide cut-and-paste instructions
to proceed.

On Solaris, where /bin/sh is actually sh, not bash, some expressions
like $() are not supported.  Git's own scripts on this machine are
configured to use "#!/bin/bash".  Change our shebang line to

  #!/usr/bin/env bash

which should work almost everywhere.

Fix logic to call check_size for all updated index entries, not just
those whose mode changed.

Check blob and tree object sizes to prevent large objects from entering
the repository.  The default limit is 1024 KiB, but it can be set with

  git config hooks.MaxObjectKiB $KiB

locally, or disabled by using 0 KiB.

Keep all mode-related check helpers in the "mode_" namespace.

Check new files and files whose mode changes to verify that each file
mode matches the content of the file.  The mode of a file must be
executable if and only if the file looks executable (name ends in a
Windows executable extension or content starts with a shebang line).

Stop processing the commit message text at a "diff --git" line.  Such
lines occur when the user commits with the "-v" option.  We should not
check the patch content; it will be stripped by Git anyway.

Commit "Reject leading TABs" added use of shell syntax of the form

  $(case "..." in
     a) ... ;;
    esac)

Some shell implementations fail to recognize that the ')' in the case
label is not the end of the '$(' expression.  Work around the problem
by moving the case block into a separate function outside the '$()'
expression.

Our style guidelines do not permit indentation by TABs.

Store a backup copy of the input message.  Remove the backup only on
success.  On failure, tell the user how to use and edit the message
without having to enter it from scratch.

These hooks perform some basic commit-time checks to keep history clean.