git tag --contains : avoid stack overflow

In large repos, the recursion implementation of contains(commit,
commit_list) may result in a stack overflow. Replace the recursion with
a loop to fix it.

This problem is more apparent on Windows than on Linux, where the stack
is more limited by default.

See also this thread on the msysGit list:

	https://groups.google.com/d/topic/msysgit/FqT6boJrb2g/discussion

[jes: re-written to imitate the original recursion more closely]

Signed-off-by: Jean-Jacques Lafay <jeanjacques.lafay@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Tested-by: Stepan Kasal <kasal@ucw.cz>
Touch up the comments in the 'tag-contains' branch

Thomas Braun pointed out several documentation shortcomings.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'tag-contains'

This topic branch addresses out-of-memory errors in particular on
Windows, where the default stack space is not very large.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>