[PATCH v5 00/14] Add interpret-trailers builtin

This patch series implements a new command:

        git interpret-trailers

and an infrastructure to process trailers that can be reused, for example in "commit.c".

1) Rationale:

This command should help with RFC 822 style headers, called "trailers", that are found at the end of commit messages.

(Note that these headers do not follow and are not intended to follow many rules that are in RFC 822. For example they do not follow the line breaking rules, the encoding rules and probably many other rules.)

For a long time, these trailers have become a de facto standard way to add helpful information into commit messages.

Until now git commit has only supported the well known "Signed-off-by: " trailer, that is used by many projects like the Linux kernel and Git.

It is better to implement features for these trailers first in a new command rather than in builtin/commit.c, because this way the prepare-commit-msg and commit-msg hooks can reuse this command.

2) Current state:

Currently the usage string of this command is:

git interpret-trailers [--trim-empty] [--infile=<file>] [(<token>[(=|:)<value>])...]

The following features are implemented:

        - the result is printed on stdout
        - the [<token>[=<value>]>] arguments are interpreted
        - a commit message passed using the "--infile=file" option is interpreted
        - if "--infile" is not used, a commit message is read from stdin
        - the "trailer.<token>.key" options in the config are interpreted
        - the "trailer.<token>.where" options are interpreted
        - the "trailer.<token>.ifExist" options are interpreted
        - the "trailer.<token>.ifMissing" options are interpreted
        - the "trailer.<token>.command" config works
        - $ARG can be used in commands
        - ditto for GIT_{AUTHOR,COMMITTER}_{NAME,EMAIL} env variables
        - there are some tests
        - there is some documentation

The following features are planned but not yet implemented:
        - add more tests related to commands
        - add examples in documentation
        - integration with "git commit"

Possible improvements:
        - support GIT_COMMIT_PROTO env variable in commands

3) Changes since version 4, thanks to Eric:

* many small functions became 'static inline' instead of just 'static' 
* alnum_len() has an "int len" parameter instead of "size_t len"
* some redundant comments were removed
* some "const" have been added
* some "switch" replaced some "if ... else if ..."
* some string related functions have been removed from strbuf.{c,h}
* some memory leaks have been fixed (but see below)
* the refactoring patch 11/17 was squashed into a previous patch
* the documentation patch was improved

4) TODO from previous review:

- check for memory leaks
- add test for empty token

Christian Couder (14):
  Add data structures and basic functions for commit trailers
  trailer: process trailers from file and arguments
  trailer: read and process config information
  trailer: process command line trailer arguments
  trailer: parse trailers from input file
  trailer: put all the processing together and print
  trailer: add interpret-trailers command
  trailer: add tests for "git interpret-trailers"
  trailer: if no input file is passed, read from stdin
  trailer: execute command from 'trailer.<name>.command'
  trailer: add tests for trailer command
  trailer: set author and committer env variables
  trailer: add tests for commands using env variables
  Documentation: add documentation for 'git interpret-trailers'

 .gitignore                               |   1 +
 Documentation/git-interpret-trailers.txt | 132 +++++++
 Makefile                                 |   2 +
 builtin.h                                |   1 +
 builtin/interpret-trailers.c             |  36 ++
 git.c                                    |   1 +
 t/t7513-interpret-trailers.sh            | 262 ++++++++++++
 trailer.c                                | 658 +++++++++++++++++++++++++++++++
 trailer.h                                |   6 +
 9 files changed, 1099 insertions(+)
 create mode 100644 Documentation/git-interpret-trailers.txt
 create mode 100644 builtin/interpret-trailers.c
 create mode 100755 t/t7513-interpret-trailers.sh
 create mode 100644 trailer.c
 create mode 100644 trailer.h